When disconnecting from one server and connecting from another, sometimes the chat from the first server merges on with 2nd server. Note that I have seen this on multiple computers with different OS's.
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pokechu22
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This happened to me today, but with Hypixel and another server. I was on the other server and the chat was merged. Then i went AFK, and for some reason the Hypixel AFK system enacted while i was afk on the other server, and moved me to Hypixel's afk room, with the chat still merged. The game literally moved me from one server to another without me doing anything. Note that i was on 1.16.5 and i am on a mac.
I'm pretty sure this is a duplicate of MC-74984; does that describe the (very weird) behavior you're talking about? Or are you connecting all the way and then disconnecting?