When Better Slabs is used in conjunction with Better End (https://github.com/paulevsGitch/BetterEnd), all of Better End's slabs excluding Violecite Slab and Violecite Brick Slab render as top or bottom slabs when they are supposed to render as vertical slabs. Which one they render as seems to depend on which face of a block they are attached to: upper for north and west, lower for south and east. They also inflict improper face culling on adjacent blocks. Their collision is still correct. This issue was tested with the latest versions of both mods.

The issue presented with sulphuric rock slabs. All of these were attached to the end stone block's sides, and should be rendering as vertical slabs placed alongside it. In addition, if a block were to be placed in the corners of this arrangement, its faces along the slabs would also be culled.

The issue on Better End's page. paulevs believes the issue to be on your side of things.
paulevsGitch/BetterEnd#47

I think I found a reason and, probably, a solution for it