no matter what I try even jumping at the build limit of 320 and fell to -60 with one water block aslo failed at -59. I can't get the xbox achievement on windows 10. I tried both a slime and a water block at the end but both failed. it seems to be bugged. I got several other achievements on the same world before and after so I know cheats are still disabled.

video
https://youtu.be/1zv3xs0l3EY