I'm not sure if this is a bug, a new design choice or a random test, but over the last few days when a new message comes in there is an orange 1 over my avatar at the top of the page and over the envelope icon on my sold orders page. That is all perfectly normal. The problem is after I respond to the message the new message indication number remains (or it's a new number popping up). When checking for a new message all that is there is my response, showing in dark type as an unread message. I actually have to open my own response and scroll through as if reading it to get it marked as read and have the orange 1 disappear.
Haven't seen that one yet. Try moving to another place in your shop to see if it goes away.
It's annoying me to no end my sympathies hope it stops soon