Apple Wallet pass balance update delay

Question

We have implemented an API service in .NET that sends background notifications to Apple Wallet to update its balance, which in turn calls below to APIs of our service:

1. webServiceURL/version/devices/deviceLibraryIdentifier/registrations/passTypeIdentifier?passesUpdatedSince=tag
2. webServiceURLversion/passes/passTypeIdentifier/serialNumber

Service sends a push notification to api.push.apple.com:443 using HTTP/2 protocol and uses Pass Type ID with NFC Certificate for auth, with success every time and the wallet invokes the above two APIs hence balance gets updated.

However around 30% of the time, pass update happens with a delay; and the delay keeps on increasing like 1 min, 2 mins, 10 mins, 1 hour, and so on, and gets reset.

I tried to add a header apns-expiration with the value UtcNow + 5 mins in epoch format in the push notification request, but it did not work.