WNS Push Notifications sometimes not received

[OleSetnes]

We are experiencing issues with Windows Push Notifications (WNS) not being delivered to our application endpoints. The push notification is sent successfully from our server, but the response header indicates the message is dropped due to a blocked channel.

Here are the relevant response headers from the WNS server:
X-WNS-MSG-ID: 2C9C70339C1E494
X-WNS-STATUS: dropped
X-WNS-ERROR-DESCRIPTION: Blocked channel URL
X-WNS-DEBUG-TRACE: AM3PEPF0000624D
MS-CV: j0Q59yxL0UmSaBRNeOmW8A.0
MISE-CORRELATION-ID: a2cf4196-f380-41bf-be19-a50c82213157

We are using the WNS service directly via HTTPS POST requests. The push tokens were obtained using standard client-side registration, and the tokens are current as far as we know. However, messages to certain channels are being dropped with the above error.

Usually we first have a Push Notification that is sent without errors but it is not received by the endpoint. The next push notification we send receives this error in the responce header. Perhaps the first notification is somehow stuct in the 'system' and that is why the following notifications can't be sent.

Reregistering the service on the receiving endpoint solves the problem so it is not likely that communication is blocked by firewalls/proxies and that the endpoint cannot connect to the notification services.

The endpoint doesn't know that it didn't receive the notification so it cannot reregister which would fix the problem. Is there a way for the endpont to check if a channel working correctly?

Steps to Reproduce:
Send a push notification using WNS to a previously registered channel URI.
Observe the response headers showing the status as "dropped" and error description as "Blocked channel URL".

The problem is usually seen after 2-3 days or more. This is for now just a test setup so we are deffinitely not overloading the notification frequency limits. The notiification that is not received and the notification that receives the Blocked channel error are more than a minute apart.

Expected Behavior:
The message should be delivered to the client device if the channel is valid and active.

Actual Behavior:
The message is sometimes dropped by WNS with error Blocked channel URL.

It works with most PCs and the above error is sometimes seen on a specific Windows 11 PC (23H2, build 22631.4460).