Just happened to me again. Starting in the mobile skin from
https://www.flyertalk.com/forum/subs...n&folderid=all
I clicked on a thread with new messages via the "first new post" link and read the two new messages.

I then went back to the subscriptions page and noticed the thread had not been updated to reflect all the messages had been read. Forced refresh, no change. Cleared cache and forced refresh, no change. Clicked on the thread again and it showed me the same two messages as if they were unread.

Possibly of note: the first unread message was N+1 where N is an exact multiple of 15, which is my default page size (of course the page size only really comes into play on the desktop skin without infinite scroll, but even in infinite scroll it still keeps track of page numbers).

This reinforces my vague recollection (in that I haven't kept notes on details but this seems to fit with my memory) that the problem tends to occur when there are multiple new pages of messages to be read, almost as if there is a "last read" tracker that remembers the last post number that you read, but that for some reason it sometimes only works on the first page and then stops recording when you cross over a page boundary (and maybe my current experience suggests it is also broken if you begin reading exactly at a page boundary)

Anyway just speculating (black box QA is fun) -- could be a total red herring.