I found a similar problem with iOS 13.1.x where all CoreData with CloudKit sync does not work unless let the app go back to foreground again.
Reported with: FB7400379
Handling of silent notifications has been changed a bit in iOS 13. To receive silent CKQuerySubscription notifications in iOS 13 and tvOS 13, the
alertBodyparameters of the
CKQuerySubscriptionmust not be set. After changing this, everything worked as before.