It seems this mod also adds vertical slabs, but the problem is that horizontal slabs magically become vertical when placed; making it impossible to have normal horizontal slabs. Breaking them somehow magically converts them back to horizontal ones in item form. Manually converted vertical slabs correctly place and drop as vertical slabs. Not only do they look vertical, but water can fall through them like vertical slabs too, and Jade identifies them as vertical while they're placed.

MineCraft: 1.18.2
Forge: 40.3.0
DoubleSlabs: 1.18-5.0.0