I was following a basic doom clone tutorial yesterday when I ran into a problem where the zombies, displayed with a Sprite3D under a KinematicBody node couldn't be seen at most angles.

When I continued anyway and added a SpatialMaterial it turned into a white square both in in the editor and in game.
I made another project to see whether it would happen again if there was nothing else and it did, but I retried it today and now it displays normally. However, it still doesn't work on the original doom project, despite removing and reimporting the source image and the fact that both projects use the same image.