I use since many years the shared inbox for different mail accounts in the k9 mail app. Since a few weeks there are no new mails in the shared folder. To read the new mail I have to go to the separate mail folder. In the overview, the app shows me the new mail in both folders (see attachment). After I read the mail, it also appears in the shared folder.
I hope the problem is understandable. I use K-9 Mail Version: 5.600 and Android 11 (Pixel 4a).