I play my brother's minecraft LAN world for some time.

I leave.

I join.

Inventory empty, all player data reset.

I check player data files.

There is only one relating to my account, my previous player file with the data for all my stuff has vanished completely leaving just thew new empty data file so I can't recover anything by renaming the files

If it means anything, the host has loads of data files that are seemingly created anew every time he save & quits