Players using local multiplayer feature lose inventory, location, permission, and spawn point data. Any players that rejoin the game as local co-op are set back to world spawn as if they have never played on the map.
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This bug has been reported to our internal bug tracker for further testing and a fix. It is scheduled to be fixed in one of the future updates (no specific date can be provided).

I am also experiencing this issue.

Having the same issue on Xbox on the better together version. I play 4 way split screen and it randomly does this to players 2-4.

I have the same issue, but in single player as well. I'll have just built a castle, saved and exited the game, loaded up the game and castle was gone!
Also, when loading profile on another Xbox one, and transfer my world that I had been working on. After loading up the map, half of everything I built is gone and it's like it loads from a later save..

Attention! I have recently accidentally found a temporary workaround for this issue. You will not have this issue if you play without an internet connection (Ex: Unplugging Ethernet cable)

Hey Joseph, do you mean you unplug your internet cable, or just set the game to offline?

I updated my previous comment to answer this question.

my game is still having this issue plz fix

Still having the local Co-op issue with spawning my girlfriends character with no inventory, no saved character skin, and at maps original start point.

My kids are having this exact problem; they play on local co-op in split screen, everything is fine and they sign out, but the next time they sign in the second player has lost all of his gear and he's at the spawn point. Is there a fix?

As of 8-10-2020 this is still an on going issue. I'm not sure why it is marked as resolved because it clearly is not. The work around suggested does not apply, since it happens both without any internet connection and when online playing in realms too.
My girlfriend and I have had to get in the habit of traveling back to world spawn, taking off all our gear and everything out of our inventories, and putting it in a chest before we exit each others games. Then it is a long process of recreating our skins when we log back in, gathering everything we had on us, and traveling back to whatever area we were working on.
I very much would like to up vote this, but since it was marked as resolved for some reason I have my own submission here for anyone who would like to vote for it. https://bugs.mojang.com/browse/MCPE-55649

I’ve been having this problem even in 1.17, where I had a full set of diamond armor on a friends shared multiplayer world (not realm), and I logged in one day to spawn 30 blocks above spawn without any items in my inventory. I just created more armor, and made it even better, but then a few days later I got my original inventory back. This issue most definitely has not been resolved.

This is still going on for me just yesterday I lost a whole shulker box filled with good loot about 3 days ago on my friend's multiplayer realm bedrock 1.18 and when I went through the end portal my game disconnected. I rejoined and lost only that 1 shulker box and I have no clue why because I just put it in a chest during that time and when I check all my chests it is still missing.
Experience as well with players on PC and Xboxone using LAN to connect.