Just to close the circle here, a fix here is massively extremely unpleasant, backend never actually counts specifically how many posts you have unread, it relies on a delta of highest_post_number and the last post_number you visited in topic users.
If I had to put a number on this, if my only job was to fix this bug it would take me multiple months to fix and I would have to invent a custom data store to get it going. Running a count(*) query on every unread topic is off the table.