With Minecraft 1.12, it's no longer possible to get stacked potions using /replaceitem command.
When I type "/replaceitem entity @p slot.hotbar.0 minecraft:potion 64", the chat says : "The number you entered is too big (64), it must be at most 1"

This bug is unacceptable, for mapmakers and others.
It's not in 1.12 patch note.
Please fix it quickly