Tested versions

• Reproducible in all 4.x versions (4.0.stable, 4.4.1.stable, 4.5.beta3)

System information

Godot v4.5.beta3 - Arch Linux #1 ZEN SMP PREEMPT_DYNAMIC Thu, 17 Jul 2025 21:05:15 +0000 on X11 - X11 display driver, Multi-window, 1 monitor - Vulkan (Forward+) - integrated Intel(R) Iris(R) Xe Graphics (RPL-P) - 13th Gen Intel(R) Core(TM) i9-13900H (20 threads) - 31.04 GiB memory

Issue description

In the Compatibility renderer, when settings a MeshInstance3D's shadow casting setting to Double-Sided, the mesh itself will be rendered without backface culling.

Steps to reproduce

1. Add a MeshInstance3D with a PlaneMesh
2. Set Cast Shadow to Double-Sided

Minimal reproduction project (MRP)

double_sided_shadows.zip