Steps to reproduce:
Build a nether portal.
Enter the nether portal.
Fly around in the nether for a bit (ideally with spectator mode so you can be faster).
❌ "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.
Related issues
Attachments
Comments

Thank you for your report!
We're tracking this issue in MC-90683, so this ticket is being resolved and linked as a duplicate.
If you would like to add a vote and any extra information to the main ticket it would be appreciated.
If you haven't already, you might like to make use of the search feature to see if the issue has already been mentioned.
Quick Links:
📓 Bug Tracker Guidelines – 💬 Community Support – 📧 Mojang Support (Technical Issues) – 📧 Microsoft Support (Account Issues)
📓 Project Summary – ✍️ Feedback and Suggestions – 📖 Game Wiki

@ManosSef: I found that bug report too, but I assumed that it only occurs in that report with boats or end crystals or something, whereas here it gets spammed all the time in the nether. If it is indeed the same bug, it would probably be helpful to include the nether spam in MC-90683.