4.2.5 has removed the obsolete thread marking option to mark read by cookies. It now marks threads read the correct way, which is database marking.
"New Posts" is ONLY supposed to show you threads with unread posts - otherwise they are not "New". Once read, they disappear from the list.
If you want to see all updated threads from the past 24 hours, use Today's Posts, which lists unread in bold and read in normal text.New Posts is supposed to show you threads with unread posts since last visit.
It is split into two parts...the top part is actual unread posts since last visit. Below that is a bar that states:
"The content below has not been updated since your last visit but may still contain unread items."
These will go back the seven days.
The old cookie based system was inaccurate and obsolete. It doesn't recognise read posts across devices. Back in 2003 when everyone used a single computer and there were no smartphones, that was fine. In the modern era, its behaviour is almost considered a bug. Removing it IS a good move because it didn't work as people expected and was the source of many tickets and complaints believing it to be broken.
Once a thread has been "read" then it will no longer appear in the New Posts list. If anyone wants a list of all threads that day, read or otherwise, use "Today's Posts", which shows unread in bold and read in standard type.