Apple Mail – number of unread emails doesn't update

In Apple Mail, the number of unread emails doesn't change after unread emails have been opened. I have to shut down the Mail app and open it again for it to update.

I started seeing this quite a while ago, long before updating to macOS 26.

Any suggestions what could be causing it or how to solve it?

The email account in question is an iCloud account.

[Edited by Moderator]