There are some issues with MDM APNS push commands and magic push tokens registrations since 29.12.2020.
Between 29.12.2020. and 2.1.2021 almost all iPhone devices did not respond to mdm push notifications
some devices that renewed device identity certificate and registered new magic push token in check-in process did not respond to push notifications for weeks, then some of them started to work again.
APNS response for push notification for some devices started to return status 410 (unregistered) then after few days again to 200 status (ok) but devices do not respond to push notifications.