Specs:

• MC Version: 1.12
• C&B Version: 14.11 & 14.12
• Do You have Optifine: No
• MCMP Version: 2.4.1

Slabs made of bits don't allow to step on the next block, if the latter is higher by half a block, like in this picture.

I tried hollow and filled slabs, and I built them using both bits and the diamond saw.
I'm sure that above me there isn't anything and vanilla slabs are working.

I hope this isn't a big problem, your mod is one of my favorite ones for building, keep the awesome work!

You most likely have another mod installed that's changing the player's stepHeight to 0.5 instead of 0.6, I would recommend making sure that all your other mods are up-to-date, if that fails to fix the issues you can try to isolate which mod it is that is making the change and report the issue to the mod developer.