Since I reported that at second of september at GN, got asked if it still works after some back and forth at first of october and its still there, I post it now here:

There is a bug with the channel you speak in inside the chat tabs.
Easy to check: go ingame, make a guild-exclusive chat tab to speak in guild for guild hunts. Than go to chat settings and allow to show "hits against me" to know about something starts attacking you - bam you speak in say channel suddenly you can't even read, instead of going on speaking in guild.
I got told by people than, that I can use the chat prefix for that single tab - but that was after long discussion with many "believers" proven to be not true, prefix works always only for the window, not for the tab. Only exclusive tabs work as mode-setter for the channel I speak in.