Flipbook based Character keeps changing planes

So I’m building a 2d game, in a 3d environment, not 2.5d, just planes with invisible boxes to walk on and create navigation paths.

However I keep having an issue where my Player Character keeps moving forward or backward on the plane, and is able to walk past enemies, sprite based obstacles, or worse of of the invisble box paths that I’ve made for him altogether.

I’ve tired locking his movement to a plane but it doesn’t seem to do anything.