When two different slabs are vertical and used in corners, it shows as invisible. I have only this mod on.

Forge: (1.16.5) 36.0.42
DoubleSlabs: (1.16.5 / April 13th upload) 1.16-3.5.0rc1

Apologies for the delayed reply, I believe that this should now be fixed in the latest version!! For now it is only for 1.16 but I will be backporting to older versions as soon as I can :)

Before I place the vertical slab.

Other photo

and this