Mobs are continuinally spawning on bottom 1/2 slabs (placed on top of a solid block) and now with 1.14, it appears that mobs can also spawn on top of chests.

Should be impossible for mobs to spawn on either. Single player worlds seem fine. The issue is on realms specifically.

Validated by creating a full perimeter at high light levels with bottom 1/2 slabs/chests on the perimeter >24 blocks & < 54 blocks away from the player in a test realm.