There seem to be a few problems with stone, which all may be related to the fact that there appears to be two types of stone that are not compatible for some reason.
1. Stone smelted from cobblestone will not stack with stone mined with a silk touch pickaxe. To duplicate, take partial stacks of stone smelted from cobblestone and stone broken with a silk touch pickaxe and try to combine them into a single stack; you won't be able to. This has been happening for some time now, well before the update.
2. With the 1.13 update, stone smelted from cobblestone cannot be further smelted into smooth stone. I've tried numerous fuel sources: coal, kelp blocks, lava buckets. None of them will work. However, stone mined with a silk touch pickaxe will smelt into smooth stone with no problem.

It seems that the game is treating stone that has been smelted from cobblestone and stone mined with a silk touch pickaxe as two different types of stone. Stone should be stone, shouldn't it?