When trying to walk from a soul sand block onto a slab (any material) you can not actually walk on it. You instead get stuck on on the edge of the two blocks.
Linked issues
duplicates 1
Comments 2
This same affect applies with stairs as well, and with that being said, this report is a duplicate of MC-42726, which was later resolved as, "Works As Intended". This "issue" that you are experiencing actually functions how the developers intended it to. Please see the parent ticket for more information.
Works as intended. When standing on soul sand, you actually sink into it by 1/8 of a block, so going up onto a slab would be a height of 5/8 of a block, which is too high to make without jumping.