Mod version: 0.45.2
Forge version: 36.0.22

[Render thread/WARN]: Unknown custom packet identifier: vehicle:play

This is occasionally printed to the log, mostly when joining a world or switching dimensions I think? I would need to go and reproduce it some more to be sure.

I'll look into it. The message is printed due to a packet being unhandled. However all of my packets acoording to my code mark the packets as handled.