I use the "View new posts" function on several forums, but have a problem.
If I visit one of these forums while searching for something, then I must either use "view new posts" there and then, or the list of new posts gets truncated to that time.
Alternatives would seem to be:
Look through each forum / subforum manually
Turn off cookies

Am I missing a better way?

When your list is limited, there is an option on the lower-right portion of the list that says

You can use that to show more topics.

right under my nose...