I’ve had this problem for a long time, apparently it’s something that is difficult to fix but does have an effective work around solution. Here’s what one of devs said on Github:
Also, if you add a 3D object (cube for instance) in the scene’s objects, the bug does not appear. Tilemap rendering when using a light · Issue #7091 · 4ian/GDevelop · GitHub
I tested this and it works. Just and a 3d box object, no need to add it to the scene.