Minecraft v1.15.2
Forge v31.2.36
Double Slabs v2.11.8

I cannot combine two vertical slabs, such as from quark or other mods, together to make a double slab. A normal horizontal slab has to be used to convert the quark vertical slab into a double slab block.

Also, if a quark vertical slab is converted into a double slab block, the break animation is off.

The mod Bamboo Blocks is also missing compat for it's vertical slabs.

Thanks for the report and apologies for the delay, this is finally fixed in the latest version I believe :)