Please include the following in your bug report:

Version of emscripten/emsdk:
Please include the output emcc -v here
emcc (Emscripten gcc/clang-like replacement + linker emulating GNU ld) 3.1.23 (5ae63ce)
clang version 16.0.0 (https://github.com/llvm/llvm-project 8b587113b746f31b63fd6473083df78cef30a72e)
Target: wasm32-unknown-emscripten
Thread model: posix
InstalledDir: /home/terop/cvs/emscripten/emsdk/upstream/bin

Failing command line in full:
If this is compile or link-time failure please include the full failing command
along with its entire output.

Setting up EMSDK environment (suppress these messages with EMSDK_QUIET=1)
Adding directories to PATH:
PATH += /home/terop/cvs/emscripten/emsdk
PATH += /home/terop/cvs/emscripten/emsdk/upstream/emscripten
PATH += /home/terop/cvs/emscripten/emsdk/node/14.18.2_64bit/bin

Setting environment variables:
PATH = /home/terop/cvs/emscripten/emsdk:/home/terop/cvs/emscripten/emsdk/upstream/emscripten:/home/terop/cvs/emscripten/emsdk/node/14.18.2_64bit/bin:/home/terop/.cargo/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games:/usr/local/games:/snap/bin
EMSDK = /home/terop/cvs/emscripten/emsdk
EM_CONFIG = /home/terop/cvs/emscripten/emsdk/.emscripten
EMSDK_NODE = /home/terop/cvs/emscripten/emsdk/node/14.18.2_64bit/bin/node
make -j 8 -f Makefile.LinuxEm
make[1]: warning: -j8 forced in submake: resetting jobserver mode.
make[1]: Siirrytään hakemistoon ”/home/terop/cvs/GameApi/GameApi”
Deps GameApi_low.cc -o objs/GameApi_low.d
Deps GameApi_main.cc -o objs/GameApi_main.d
em++ GameApi_main.cc -o objs/GameApi_main.bc
em++ GameApi_low.cc -o objs/GameApi_low.bc
echo em++ libGameApi.bc
em++ libGameApi.bc
em++ -shared objs/GameApi_gui.bc objs/GameApi_gltf.bc objs/GameApi_diag.bc objs/Pieces.bc objs/Map.bc objs/VectorTools.bc objs/Effect.bc objs/Buffer.bc objs/Intersect.bc objs/Bitmap.bc objs/Effect2.bc objs/Shader.bc objs/VolumeObjects.bc objs/Tree.bc objs/Event.bc objs/ObjectCreator.bc objs/IntersectObject.bc objs/Graph.bc objs/Widgets.bc objs/Category.bc objs/Plane.bc objs/Coordinates.bc objs/Triangle.bc objs/Game.bc objs/Editor.bc objs/Web.bc objs/Functions.bc objs/ShaderExpr.bc objs/Font.bc objs/FontEditor.bc objs/GameApi.bc objs/GameApiTest.bc objs/Games.bc objs/HierarchyObject.bc objs/Physics.bc objs/GameApiTest2.bc objs/State.bc objs/Serialize.bc objs/KeyFrameEditor.bc objs/Parser.bc objs/FreeType.bc objs/FreeType2.bc objs/VertexArray.bc objs/StateChange.bc objs/DistanceObject.bc objs/GameRunner.bc objs/GameApi_an.bc objs/GameApi_bm.bc objs/GameApi_co.bc objs/GameApi_dr.bc objs/GameApi_ev.bc objs/GameApi_ex.bc objs/GameApi_fbo.bc objs/GameApi_f.bc objs/GameApi_fn.bc objs/GameApi_fnt.bc objs/GameApi_gr.bc objs/GameApi_lay.bc objs/GameApi_li.bc objs/GameApi_main.bc objs/GameApi_mx.bc objs/GameApi_pc.bc objs/GameApi_phy.bc objs/GameApi_pla.bc objs/GameApi_pl.bc objs/GameApi_pt.bc objs/GameApi_pts.bc objs/GameApi_sh.bc objs/GameApi_shm.bc objs/GameApi_sm.bc objs/GameApi_spa.bc objs/GameApi_sp.bc objs/GameApi_st.bc objs/GameApi_su.bc objs/GameApi_tex.bc objs/GameApi_tr.bc objs/GameApi_trk.bc objs/GameApi_tx.bc objs/GameApi_vbo.bc objs/GameApi_ve.bc objs/GameApi_vo.bc objs/GameApi_wmod.bc objs/GameApi_wv.bc objs/GameApi_vx.bc objs/GameApi_cut.bc objs/GameApi_in.bc objs/GameApi_imp.bc objs/GameApi_plane.bc objs/GameApi_integrator.bc objs/Main.bc objs/GameApi_vr.bc objs/GameApi_low.bc objs/GameApi_dep_add_find.bc objs/GameApi_dep_env.bc objs/GameApi_dep_arr_render.bc objs/GameApi_webcam.bc objs/Math2.bc objs/GameApi_cmd_move.bc objs/GameApi_cmd_bbitmap.bc objs/GameApi_cmd_pdfield.bc objs/GameApi_cmd_bitmap.bc objs/GameApi_cmd_point.bc objs/GameApi_cmd_booleanops.bc objs/GameApi_cmd_points.bc objs/GameApi_cmd_cvolume.bc objs/GameApi_cmd_polygon2.bc objs/GameApi_cmd_fbitmap.bc objs/GameApi_cmd_polygon.bc objs/GameApi_cmd_font.bc objs/GameApi_cmd_framebuffer.bc objs/GameApi_cmd_texture.bc objs/GameApi_cmd_fvolume.bc objs/GameApi_cmd_lines.bc objs/GameApi_cmd_vector.bc objs/GameApi_cmd_mainloop.bc objs/GameApi_cmd_volume.bc objs/GameApi_cmd_material.bc objs/GameApi_cmd_waveform.bc objs/GameApi_cmd_shader.bc objs/GameApi_cmd.bc objs/cpushader.bc objs/music.bc objs/music_html5.bc objs/prerequisites.bc -o libGameApi.bc -s ERROR_ON_UNDEFINED_SYMBOLS=0 -Lhssynth2/synthlib/emscripten -lsynthlib -Lhssynth2/midiplay/midifile/emscripten -lmidifilelib
em++: warning: linking a library with -shared will emit a static object file. This is a form of emulation to support existing build systems. If you want to build a runtime shared library use the SIDE_MODULE setting. [-Wemcc]
make[1]: Poistutaan hakemistosta ”/home/terop/cvs/GameApi/GameApi”
(cd web_page; make -f Makefile.LinuxEm pthreads_both)
make[1]: Siirrytään hakemistoon ”/home/terop/cvs/GameApi/GameApi/web_page”
(cd ..;em++ -s USE_PTHREADS=1 -s PTHREAD_POOL_SIZE=30 -s FILESYSTEM=0 -g -gsource-map --source-map-base https://meshpage.org/gameapisource/ -s ASSERTIONS=0 --use-preload-cache -s NO_EXIT_RUNTIME=1 -s TOTAL_MEMORY=1750Mb -s TOTAL_STACK=20Mb -s WASM=1 --bind -std=c++11 web_page/test.cpp libGameApi.bc -s USE_SDL=2 -s USE_FREETYPE=1 -s DEMANGLE_SUPPORT=1 -L. -Lobs -I. -o web_page/web_page_highmem.html --memory-init-file 0 -s ERROR_ON_UNDEFINED_SYMBOLS=0 -s "EXPORTED_RUNTIME_METHODS=['__cxa_is_pointer_type', '__cxa_can_catch','ALLOC_NORMAL', 'ALLOC_STACK', 'ALLOC_DYNAMIC', 'ALLOC_NONE', '_set_new_script', '_activate_trigger', '_stop_music_playing', '_set_toggle_button', '_set_integer', '_set_float', '_set_string', '_set_background_mode', '_set_resize_event', 'cwrap', 'ccall', 'FT_Get_Char_Index', 'FT_Load_Gryph', 'FT_New_Memory_Face', 'FT_Render_Glyph', 'FT_Set_Char_Size', 'emscripten_fetch','emscripten_fetch_attr_init', 'emscripten_fetch_close', '_get_integer']" -s ENVIRONMENT=web,worker -s DISABLE_DEPRECATED_FIND_EVENT_TARGET_BEHAVIOR=1 -s FETCH=1 -s MIN_WEBGL_VERSION=2 -s MAX_WEBGL_VERSION=2 -s GL_SUPPORT_AUTOMATIC_ENABLE_EXTENSIONS=0 -s DISABLE_EXCEPTION_CATCHING=0)
warning: undefined symbol: $FS (referenced by $PATH_FS__deps: ['$PATH','$FS'], referenced by Mix_LoadWAV_RW__deps: ['$PATH_FS','$SDL'], referenced by Mix_LoadMUS_RW__deps: ['$SDL','Mix_LoadWAV_RW'], referenced by Mix_LoadMUS__deps: ['Mix_LoadMUS_RW','SDL_RWFromFile','SDL_FreeRW','$SDL'], referenced by top-level compiled C/C++ code)
warning: undefined symbol: _Z15add_shader_codeRN7GameApi3EnvEP10ShaderCode (referenced by top-level compiled C/C++ code)
warning: undefined symbol: _Z16find_shader_codeRN7GameApi3EnvENS_3SHCE (referenced by top-level compiled C/C++ code)
warning: undefined symbol: _Z24add_screenspace_materialRN7GameApi3EnvEP19ScreenSpaceMaterial (referenced by top-level compiled C/C++ code)
warning: undefined symbol: _Z25find_screenspace_materialRN7GameApi3EnvENS_3SMTE (referenced by top-level compiled C/C++ code)
warning: undefined symbol: _Z7add_pboRN7GameApi3EnvEP17PixelBufferObject (referenced by top-level compiled C/C++ code)
warning: undefined symbol: _Z8find_pboRN7GameApi3EnvENS_3PBOE (referenced by top-level compiled C/C++ code)
warning: undefined symbol: glAlphaFunc (referenced by top-level compiled C/C++ code)
warning: undefined symbol: glMapBuffer (referenced by top-level compiled C/C++ code)
warning: undefined symbol: glUnmapBuffer (referenced by top-level compiled C/C++ code)
em++: warning: warnings in JS library compilation [-Wjs-compiler]
em++: warning: running limited binaryen optimizations because DWARF info requested (or indirectly required) [-Wlimited-postlink-optimizations]
#wasm-strip web_page_highmem.wasm
brotli --quality=5 --force web_page_highmem.wasm -o web_page_highmem.wasm.br
make[1]: Poistutaan hakemistosta ”/home/terop/cvs/GameApi/GameApi/web_page”

Full link command and output with -v appended:
Even for runtime issues it helps a lot if you can include the full link command.
Adding -v to the link command will show all of the sub-commands run which
can help us diagnose your issue.

The basic problem is that SDL_PollEvent() is able to write outside of the SDL_Event struct. It only write a one or few bytes...
I solved a significant problem in my 3d engine using the following hack:
struct SDL_EVENT_HACK { SDL_Event e; unsigned int cc; }; SDL_EVENT_HACK h; SDL_Event &e = h.e; int val = ::SDL_PollEvent(&e);

The PollEvent call writes over that unsigned int cc; variable, so that's clearly a bug in SDL2 or clang unions.
Usually this SDL_Event struct is in stack, so if you don't have anything critical in that location in the stack, the bug will be completely invisible.