I found this while working on my mod at this line, but I can't reproduce the problem in a test mod. I'm able to start the game, but as soon as a world loads, I'm kicked:

Client disconnected with reason: Internal Exception: io.netty.handler.codec.DecoderException: Failed to decode packet 'clientbound/minecraft:custom_payload'

I've got the full repo and the log. If I omit implements PolymerStatusEffect, the kick goes away and I can join fine.
Currently I've only tested this on Singleplayer.
I can't seem to figure out where the packet is sent though :/
Thank you!

Apologies, apparently I had a bad identifier :(
cya xd