Steps to reproduce:

1. Build a nether portal.
2. Enter the nether portal.
3. Fly around in the nether for a bit (ideally with spectator mode so you can be faster).
4. "Received passengers for unknown entity" occurs multiple times in the console.

Note that in 1.21.1 it is "fzg" that throws the error message, not "gbi" as in 24w34a.