Since updating to 1.16.210 splitscreen players are not logged into their Xbox accounts. Pause menu doesn't show pictures of other players, they don't get achievements, lose their skin settings and they got entirely new data on already existing worlds causing their pets to not listen to them and loosing their inventory, ender chest content and levels.
We have encountered this issue before but it was rare and we could solve it by resetting a game but since the update it happens to every splitscreen player every single time on every single world.
I've watched other bug reports being closed and redirected to MCPE-55815 and even though it does share symptoms I think it might be different cause as linked issue is over a year old and we have this problem since 1.16.210 (update that, according to Minecraft wiki, has this in a change log: "Split-screen characters are now stored in memory and re-joining the game will keep the character that the player had (MCPE-58640)".).
This is a huge problem for us as there is no way to downgrade to older version and there is no point in progressing a game until this is fixed (as I expect a fix would cause levels and inventory loss again as players would get their previous inventory/pets/etc.)
Edit1: Players also share main account's owned skins and main player's sales with villagers (only the main player cured villagers but everyone has the sales now). Didn't notice how it worked before but I figured it might have to do with this bug as well (as I imagine that everyone should have their own prices).
Does this issue still occur in 1.16.220?