iOS Mail - Unread filter function not showing all my emails in the general mailbox

The function that allows you to filter the mails in iOS Mails on those that are not read does not display the mails received from a new mailbox that I have just created.

I've just added a new mailbox to iOS Mails. It appears in the list of mailboxes.

If I enter this new mailbox by clicking on it and activate filtering in the bottom left-hand corner on unread mail, it works correctly. I can only see unread mail in this inbox.

On the other hand, if I activate filtering on all my mailboxes from the general filtering button, I can't see the mail in this new mailbox.

And if I go to the filtering options, the new mailbox isn't in the list. So I can't activate filtering on it.

How can I get this new mailbox to appear in the list of my mailboxes in the general filtering option (which includes all mailboxes), so that I can include the unread mail received in this new mailbox in the general list?