Imagine being an 11 year old girl. You have been grinding for days to get your diamond gear. And suddenly, it just disappears.

This has been happening for at least 6 years. See MC-75595

Person A hosts a LAN game with person B joining.

Sometimes* when B joins the game, she is back at original spawn with empty inventory.

When that happens, a new playerdata file has appeared. A username lookup (https://mcuuid.net/) using the UUID of the new playerdata file returns nothing.

When using a converter (http://tools.glowingmines.eu/#load) to map the player name of B to an offline UUID, the resulting UUID matches the UUID in the new playerdata file name.

*) Cannot tell if it is random, or every 10 joins, or every Tuesday.