Slabs of different materials will not stack when placed in world

I noticed last week, and the problem has persisted, that I cannot place slab blocks of different materials on top on one another anymore when building. Two slabs of the same material still stack just fine, but you cannot place two different wood slabs on top of each other or different stone slabs or wood and stone. 

It first affected me in Realms, but it also happens in Singleplayer.