It would appear that as of the most recent hotfix update a new bug has surfaced. This allows you to load into a flat world transferred from Xbox One Edition, however once you save and quit, the game will never let you back on that world, it will just say "Syncing data" infinitely.

I have tried this with normal worlds transferred from Xbox One and have had no issues other than one singular crash out of about a dozen loads.

I almost exclusively play on custom superflat worlds so this basically makes me not even want to play the game at all. I know it takes time to fix but world generation issues like this should take top priority and I hope that this bug gets that priority.

EDIT: Okay gonna update this, so apparently this bug happens with ANY world transferred from Xbox One Edition to Xbox Bedrock Edition, it is not exclusive to superflat worlds, please fix this, it is actively annoying me that I can't play the game how I want to.