Unread Filter in iOS not working

Hi!

When I click on the unread filter at the bottom left of the Mail app on iOS it doesn’t filter the unread messages any longer.

I just get an empty mailbox as if I don’t have any unread emails when in reality I have 451 unread emails at the moment.

I am on the latest iOS 18.1 and am using an iPhone 13 mini.

Anyone having the same problem or any suggestion on what I can do to solve this problem?

Thanks in advance