I am having this exact issue and I made a bug report about it here: TextChannel's channel tab does not disappear when player's TextSource is removed from a TextChannel
Having a player leave a TextChannel that still exists is causing a lot of weird issues like being able to still see the channel and being able to type ‘ghost’ messages in it, and sometimes being locked out of General and not being able to type at all. There is no way to remove a TextChannel from your tabs unless the TextChannel is deleted for everyone.