This is from a Planet Minecraft post, so it may have already been fixed, and there are no attachments. Here it is:
I was hosting a world over lan and a friend was connected, we are in survival and we made a flying machine with a boat on it. We were both in the boat when I save and quit.
When I next reconnected, I just fell out of the sky where the boat was, everything seemed normal, but the boat was gone. When I reopened it to lan and my friend reconnected, he spawned in at world spawn and had an empty inventory, also his skin no longer appears, he is just the default alex skin.
No matter how many times we each relaunch and change networks his inventory and skin are still missing. Is there any way of getting his stuff back, and why is his skin gone?
BTW we are in the latest snapshot (20w13b)
Thats the entire post. https://www.planetminecraft.com/forums/help/javaedition/problems-with-lan-and-losing-inventory-when-disconnecting-while-in-boat-603726/ is the link to it.
It's possible that the game just was unable to save the new savestate (i.e. the other player's inventory and the newly built machine), for instance because they ran out of storage space on their PC.
It would be useful if they could provide the game log from when this happened. It can be found in
.minecraft/logs.