No, I thought I saw a pattern but yesterday it was negated.
What I can tell is that it's always for threads I'm subscribed to. So, I access them from the User Control Panel - New subscribed threads.
I just checked the ones from today. The thread 'I dream of Leica' jumped halfway into post #77.
The first post that has '(New!)' assigned is #79 and that is correct in my case.
This is the address that was used:
https://www.pentaxforums.com/forums/pentax-slr-lens-discussion/102345-i-dream...ml#post1433343
Now I have seen this happen in other threads as well. Threads that later - like just now - behaved correctly.
Some examples from the past: Takumar club, The FA Limited club
I took me a while to realise something strange was happening.
I guess this is going to be a difficult case to pin down?