The issue is in fact caused by a missing call to IncrementUpdateCount() on the render texture.
For performance reasons, cookie texture atlas isn't updated every frame for every textures (as most of them are static anyway). To tell the system that the content of a texture has changed, you need to call IncrementUpdateCount() on the render texture, the system can then check the updateCount property to see if it has changed.
However there is several things to note:
- A mistake in the code made the atlas wrongly update in the editor. We will fix this so that behavior stays consistent between build/editor.
- The documentation does not mention the need for IncrementUpdateCount so we will add a section to the light documentation to highlight that.
Additional note on the repro scene.
The script was creating a new RenderTexture each frame which is wrong because it will flood the atlas with a new texture every frame. I assume that this was some left overs from debugging tests but this should be removed for a correct behavior.