The game seems to not recognise some block "codes" ingame. When creating a new flatlands world, I typed the old ID of the full stone slab (43:8) and it successfully converted it to "minecraft:double_stone_slab:8" (making "minecraft:double_stone_slab" the actual block "code"). It didnt show the block in the list, instead there was just air, unlike in previous versions. The block appeared normally ingame, but was unobtainable through commands, no matter if it was through a command block or me typing it. I tried installing Too Many Items and it couldnt find it too.