I have 4 items in my Inbox marked as Unread. When clicking on them, they mark as Read as usually, and the counter decrements. After reloading the page all 4 items are marked as Unread again.
When clicking on "Mark all read" the expected dialog appears
and after clicking "Mark Read" the 4 items are marked Read; after reloading the page the counter indicates 4 unread posts and there are 2 of them marked as Unread
After one more reload, all 4 are marked Unread again.