Gridmap mesh custom data is removed when baking lightmaps

[otonashixav]

Tested versions

• Reproducible in v4.3.dev4.official [df78c06]
• Did not test any other versions

System information

Godot v4.3.dev4 - Windows 10.0.19045 - Vulkan (Forward+) - dedicated NVIDIA GeForce GTX 970 (NVIDIA; 31.0.15.4665) - Intel(R) Core(TM) i7-8700 CPU @ 3.20GHz (12 Threads)

Issue description

When baking lightmaps, meshes in gridmaps lose their custom data (CUSTOM0-CUSTOM3).

2024-03-08.01-46-47-1.mp4

Edit: At least MODEL_MATRIX is also incorrect, and likely many other properties, though uniforms seem ok.

Steps to reproduce

1. Open the MRP
2. Bake Lightmaps
3. Custom data disappears
4. Modifying the GridMap brings back the custom data, but clears the baked meshes resulting in modules/gridmap/grid_map.cpp:1315 - Index p_idx = 0 is out of bounds (baked_meshes.size() = 0).; baking them again makes the custom data disappear again.

Minimal reproduction project (MRP)

repro.zip