Minecraft v1.18.1
Forge v39.0.88
Extended Slabs + v2.0.0

In first-person view, the corner blocks render slightly too big when compared to other blocks. In third-person view, the corner blocks render inverted and vertical slabs render detached from the hand. The missing texture comes from Unable to resolve texture reference: #missing in extendedslabs:block/corners/"block_type"_corner in the log

I also noticed that stone type corners and vertical slabs do not have stonecutter recipes, and stonecutter recipes to go straight from the full block to the corner block without having to craft the stair first.

Thanks for the report. Will look into all of these bugs.

As a note. This is not fixed in the 1.18.2 update yet btw.