If you're using the new mailbox go back to the old one and you'll probably find them.
If you're already in the old mailbox and can't see them they've probably dropped pff the bottom of the page so delete newer messages you can see and they'll come back onto the page
You can only get to older unread ones
by deleting the newer messages so older ones show again
or via desktop site there's an option to open the next unread message
neither options are ideal
