Minecraft v1.16.5
Forge v36.2.4
Double Slabs v3.7.1
Lord of the Rings v4.6

Slabs from Lotr render incorrectly as vertical slabs.

I have this issue as well, "LotR:Renewed" have integrated Slabs.
I've noticed that any mod that has integrated verticality for slabs has a render conflict (see: cushions in "Wily Textiles" Mod)

Thanks for the issue, the LOTR slabs should now place and appear correctly when I release the next version!