This is about the 3rd time I've seen this now (in about 5 years of using notification service extension, however all the occurrences have been in the past couple of months so presumably its some recent instability).
What happens is a push is sent to the handset and is meant to get intercepted by a notification extension, except that doesn't happen - not due to an error with the app - something internally has gone wrong with the OS.
Once this starts the notification service extension can never receive pushes again until the phone is restarted after with the extension starts to receive the pushes again.
I got log from the OS when it did this and found there was an RBSRequestErrorDomain Code=5 error
[u A0F3A639-F287-4B0C-AADE-A6F547D91404:m (null)] [<private>(<private>)] Failed to start plugin; pkd returned an error: Error Domain=PlugInKit Code=4 "RBSLaunchRequest error trying to launch plugin com.theCompany.app.NotificationServiceExtension(A0F3A639-F287-4B0C-AADE-A6F547D91404): **Error Domain=RBSRequestErrorDomain Code=5 "Launch failed**." UserInfo={NSLocalizedFailureReason=Launch failed., **NSUnderlyingError=0x5faa61190 {Error Domain=NSPOSIXErrorDomain Code=111 "Unknown error: 111"** UserInfo={NSLocalizedDescription=**Launchd job spawn failed**}}}" UserInfo={NSLocalizedDescription=RBSLaunchRequest error trying to launch plugin com.theCompany.app.NotificationServiceExtension(A0F3A639-F287-4B0C-AADE-A6F547D91404): Error Domain=RBSRequestErrorDomain Code=5 "Launch failed." UserInfo={NSLocalizedFailureReason=Launch failed., NSUnderlyingError=0x5faa61190 {Error Domain=NSPOSIXErrorDomain Code=111 "Unknown error: 111" UserInfo={NSLocalizedDescription=Launchd job spawn failed}}}}
Can file a radar but just posting here first just in case anybody from Apple know what the OS error code and hence the reason.
(longer console log available)