I have the same problem. It started happening ca. 2 weeks ago and I can see the label in SoGo and Gnome’s Evolution. Evolution colors all emails with the important label in red, so now I’m looking at a wall of red emails which is quite annoying.
Any idea how I can figure out whether this is an email client on one of my devices going rogue or something misbehaving in mailcow? Maybe there is a log where I can see which login session added the label?