Lately with fabric I have been noticing bugs of where i leave a server (such as minehut), and I still get the chat messages in another server such as a privately hosted world. I can replicate the even live if needed. maybe it is caused from a client mod but I doubt it. I am not sure if both servers see my chat but i know I can only use commands of the active server. ill give a log on request.

Can you reproduce this with only Fabric API installed? Otherwise it might be another mod.

I think this is vanilla behavior, me and some of my friends have seen this happen multiple times under instances of Vanilla or OptiFine without a modloader, but it usually involves trying to connect, canceling immediately, and then joining another server.