When joining one of my single player worlds after exiting a friend's realm, I discovered that my inventory was entirely empty, as was my Ender Chest. I walked around a bit and picked up a few items, while trying to figure out what was wrong.

Eventually I quit and relaunched the game, then rejoined my single player world., Not only were my inventory and Ender Chest restored, but I was also in a totally different location from where I was when I quit. 

To diagnose the issue, I again joined my friend's Realm, then exited the realm and opened my single player world again. My inventory was once again empty (except for the three items I had picked up while looking for my items before) and I was in the same location where I had logged off before relaunching the game.

This is consistent – it's as if there are two different instances of my character in my single player world: one with all my items, and one without. They can move independently of each other, and pick up items, and their states seem to be saved separately.

Steps to reproduce:

1. Open Minecraft 1.19.60
2. Create a single player world and join it
3. Acquire some items and move to a new location
4. Exit single player world, and join another player's Realm
5. Exit realm, and rejoin single player world
6. Observe that player's location and inventory have reset
7. Quit and relaunch Minecraft
8. Rejoin single player world
9. Observe that player inventory and location are restored.