Hey, so this is a strange issue that seems to happen while a vertical slab is waterlogged and unloaded and reloaded... I have some screenshots here.

It also happened when a slab was waterlogged but then un waterlogged, when I came back those ones were also bugged like this.

VERSION: 1.18.2

MODVERSION: 1.18-5.0.0

The missing slabs are ones that I broke, those are the ones that were waterlogged and then un-waterlogged manually. The ones below were always waterlogged.