Apple mail: blue dot remains

Lately the blue dot does not disappear from mail in the Apple mail app on my Iphone 14 pro(17.4.1). This only happens when the mail is read elsewhere, then synchronized. However, despite the blue dots all still being there, if you filter by "unread", none of the email show up. The app knows they have been read, but does not clear the blue dot. Restarting the phone or close the app does not help. I end up having to read each of the emails again on the phone. Why?