Yes, they are disabled. And that is the difference between all other of my installations: they are unified. In all other 2 Macs, if I de-unified them, they show the same unread items, and if I unify here, the unread count disappears and reappears on disabling it.
Yes. A “Windows” is a must in latest macOS versions for any issue. I think it is a glitch. With unified inboxes disabled, it appears in all my 3 installations. With them disabled, disappear.