In fact, this is the expected behavior from ExtrudeShapeCustom.
Here’s an updated PG where I added some extra path points to make sure we turn with a 45° angle:
https://playground.babylonjs.com/#F82TBJ#3
Now, the top has always the same thickness, but we don’t rotate it at a right angle because once the (square) shape is rotated, its length is too small for that: we would have to scale it by sqrt(2), but only in the X and Y dimensions, not in Z. This is something that ExtrudeShapeCustom cannot know. This function simply takes the shape, makes it follow the path, and connects the points.