Some Slopes Disappearing in Level editer?

[Rare]

When I'm In one of my Custom Levels the slopes in the level and the Blocks window disappear.
I don't know how this happened and I need help.
*Yes slopes that are dissappearing are replaced with custom GFX.
* I just realized if the slope is replaced with a custom GFX It doesn't appear.

[Kley]

This might be a bit much to ask, but are you able to upload a short video clip of the error? It'd help diagnose the issue, considering that this problem is unheard of.