I know there was a previous thread on this, but have fewer search resources than usual just now. In any case, in the last thread the cause wasn’t narrowed down but I think I’ve isolated it.
I have Party chat in my “Main” tab only. As you know, the tab title turns red to signify that there are unread messages. When I am in SW or chasing events for the daily or doing Teq, I stay in my “Map” tab at the cost of not seeing party chat unless I toggle over. Frequently I find that the “Main” gets stuck as red.
I believe this happens when the party disbands while I am in the Map tab. The light green messages about “X is no longer in the party” go in Main, and then Main stays red.