I actually converted to SpongePls SpongePls [BungeeCord Forge Support]
And regular BungeeCord but it seems to have the same issue, I think this might be a Sponge/Forge issue
It shows to authenticate the user fine with no errors server-sided:
[20:29:33] [Netty Epoll Server IO #4/TRACE] [FML/]: Handshake channel activating
[20:29:33] [Netty Epoll Server IO #4/DEBUG] [FML/]: FMLHandshakeServerState: null->FMLHandshakeServerState$1:START
[20:29:33] [Netty Epoll Server IO #4/DEBUG] [FML/]: Next: HELLO
[20:29:33] [Netty Epoll Server IO #4/DEBUG] [FML/]: FMLHandshakeServerState: $ClientHello->FMLHandshakeServerState$2:HELLO
[20:29:33] [Netty Epoll Server IO #4/INFO] [FML/]: Client protocol version 2
[20:29:33] [Netty Epoll Server IO #4/DEBUG] [FML/]: Next: HELLO
[20:29:33] [Netty Epoll Server IO #4/DEBUG] [FML/]: FMLHandshakeServerState: $ModList:4 mods->FMLHandshakeServerState$2:HELLO
[20:29:33] [Netty Epoll Server IO #4/INFO] [FML/]: Client attempting to join with 4 mods : [email protected],[email protected],[email protected],[email protected]
[20:29:33] [Netty Epoll Server IO #4/INFO] [FML/]: Attempting connection with missing mods[]
[20:29:33] [Netty Epoll Server IO #4/DEBUG] [FML/]: Next: WAITINGCACK
[20:29:33] [Netty Epoll Server IO #4/DEBUG] [FML/]: FMLHandshakeServerState: $HandshakeAck:{2}->FMLHandshakeServerState$3:WAITINGCACK
[20:29:33] [Netty Epoll Server IO #4/DEBUG] [FML/]: Next: COMPLETE
[20:29:34] [Netty Epoll Server IO #4/DEBUG] [FML/]: FMLHandshakeServerState: $HandshakeAck:{3}->FMLHandshakeServerState$4:COMPLETE
[20:29:34] [Netty Epoll Server IO #4/DEBUG] [FML/]: Next: DONE
[20:29:34] [Netty Epoll Server IO #4/DEBUG] [FML/]: FMLHandshakeServerState: $HandshakeAck:{4}->FMLHandshakeServerState$5:DONE
[20:29:34] [Netty Epoll Server IO #4/DEBUG] [FML/]: Next: DONE