BungeeCord Issue

I wonder if it's something to do with the timing between the reset packet being sent and the server switch occuring...

Consider this: once the reset packet is sent, the client returns to the frozen "vanilla Forge" state. Usually, the handshake would be quick enough for no more information to get through before the handshake completes and the server switch occurs.

What should happen is:

Reset packet is sent
Handshake completes with no other Forge packets coming through
Server switches
Disco
What might be happening is that during the handshake, other packets from the original server are getting through, but the mod in question has essentially been unloaded/client mapping changed - probably from the mod ID list being sent. In fact, that's probably it, once the mod list has been sent and accepted, packets from the old server are still getting through, but the new, potentially different IDs from the objects could be causing packets from the old server being translated incorrectly.

We should stop all packets from the old server after the mod list has been exchanged and see if that works... once I get time to look at it.