iPhone push notifications not working

I've noticed recently that my iPhone XR no longer seems to be receiving notifications from some apps. Specifically, the Microsoft Authenticator app and the FastMail email client. I think that what these two apps have in common is that they're both push notifications. (I think there are other apps that aren't doing push notifications, but those two are the obvious ones, and the easiest to test.)

Push notifications from first-party Apple apps (like the Messages app) are still working.

I know that my home network isn't the issue, since my iPad continues to receive notifications.

I don't have any "focus" modes turned on.

I have already turned my phone off & on a couple of times, to see if that would clear it.

(And I force-quit all apps before the restart.)

Any ideas as to what else I could try? Thanks.