Here is something I tried with one of those threads with which I am still having problems.

If I attempt to use MyFlyerTalk to take me to the first unread posted content in that thread, it does not remember what I last viewed.

However, once that thread had another “page” added to it, if I click on Last Page to take me to the last page of the thread and then choose View First Unread, it then works properly.

I hope this workaround helps towards finding an overall solution to this random yet annoying problem...