Steps to reproduce:

1. Create a new world on a server.
2. Log in to that server, aquire some items in your inventory.
3. Stop the server.
4. Copy the world from server to single player folder.
5. Open that world in single player.
6. You will spawn at 0, 0, 0 coordinates suffocating in the wall with empty inventory. (Your player data in world/playerdata folder resets).

Was able to confirm that versions 1.20.1, 1.20.2 do NOT have this issue.
Reproduced both on 1.20.3 and 1.20.4.