Minecraft v1.15.2
Forge v31.2.36
Double Slabs v2.11.8
Lord of the Rings Renewed v2.1.2

The mod just had an update that rewrote their vertical slab placements, so now whenever I try to place a lotr slab vertically, this happens everytime:

Apologies for the delay, this should now be fixed in the latest version :)