using a behaviour pack can disable the achievements most of the time. check if you loaded it with any behaviour packs enabled!
Hi!
You need to make sure that all the blocks of water aren't flowing water, and are still water. If you place one water bucket at the top it will produce flowing water, even if you place one at the bottom. My best suggestion is to find some kelp and fill the entire elevator with it, then destroy the kelp, this will create still water throughout!
These can sometimes be artistic changes, but I think this is a bug.