We already found the cause of the problem.
We are using the UIDevice.systemName in determining the APNS URL.
For iOS15 devices, we got iOS as the UIDevice.systemName.
For iPadOS14 devices, we got also iOS as the UIDevice.systemName.
But for iPadOS15 devices, we got iPadOS as the UIDevice.systemName.
Since on our server-side, we only checked for iOS to determine the APNS URL, the push notification was not registered.
I hope this will also help others who have the same problem as ours.
JulietBaylon
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Hello, we also have the same issue.