I may have something similar but may also have an idea as to why.
When I reply to a post, then click on the hamburger menu, it will show Unread (1). Clicking on that tells me I have no unread messages. (The false update message.)
Is a routine that checks for new messages and then shows the unread message counting my reply as a new message? I haven’t seen this happen when other users post or reply, just with my own.