Thanks for the question. To troubleshoot this issue, I'd like you to temporarily create the "Unread" favorite mailbox:
Make a favorite mailbox. Favorites appear at the top of the Mailboxes list. To add one, tap Edit while viewing the Mailboxes list. Tap Add Mailbox, then select the mailboxes to add. You’ll also get push notifications for your favorite mailboxes.
When creating a favorite mailbox "Unread" will be an option. After adding this mailbox, check to see that there are not any unread emails.