Repro:

1. in Compilers.sln, open Syntax.xml for C# and add a new syntax node (duplicate an existing one and change its name)
2. in the Error List, there should be diagnostics because the added public APIs for the new syntax node have not been recorded, but there are not.
3. from the command-line, invoke build.cmd -runAnalyzers and the public API diagnostics are correctly produced

Similarly, if you enable nullability tracking (add #nullable enable at the top of the shipped public API file), the diagnostics to record the annotation are produced from the command-line, but not in the IDE (see PR #41000).

FYI @jasonmalinowski