Our IT team under @AnthonyG has made a workaround regarding the concern of new posts disappearing from the New Posts list when you've read it. When you click new posts for the first time, you will see this notice. If you forgot to take note of it, here it is:

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.