Server can put client in an illegal state when sending custom resource pack

Sending the MC|RPack packet at a particular point in the login puts the client in an apparently illegal state. The client immediately returns to the main menu, but remains fully connected to the server. The player entity spawns normally and just stands there. If the client attempts to connect to the server again, they get the error message "You are already connected to this server". There does not seem to be any way to play the game or disconnect from the server. The client can even join another server while remaining connected to the first one.

To make this happen, I sent the MC|RPack from BungeeCord's PostLoginEvent, which appears to fire immediately after changing to the "play" protocol state. Hopefully that is enough info to reproduce this. If not, let me know and I will try to narrow it down more.

I made this bug private in case there is any concern about servers using this maliciously somehow.