The double slabs textures are not working when I'm using any shaders, I did two tests:
That's with SEUS renewed (on/off)

And that's with BSL 7.2 (on/off)

I'm also getting the OpenGl Error: 1281(invalid value)
All my drivers and mods are up to date.

Apologies for the delayed response but I believe this is 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 :) Thank you for your patience