Example:
https://www.dropbox.com/s/7mn6yuq03l4khi9/Screen%20Shot%202019-12-26%20at%205.35.37%20pm.png?dl=0
i.e. chat is without spaces or encoding and starts with a 6, probably so encode leftover.
This is because of the way the polychat bukkit client handles the chat, It would probably be easiest to check in the forge mod if the message has come from the forge mod or the bukkit plugin and parse it differently.
I would also move this to: https://github.com/DemonScythe45/polychat-client/issues
