Issue with Offsite Payments App RSU(Refund Session URL's) Callback.

[GkPayments]

My query is around the RSU (Refund Session URL's) callbacks in an offsite payment app.

I have 2 offsite payment apps, configured in a similar manner. Both of these have RSU and PSU callbacks added, the URL's are the same.

I'm only receiving RSU callbacks for only one of the apps, the other app does not receive any RSU callbacks.
The flow of the second app, is an order is placed with status 'Payment Pending' and the converted to 'Partial Paid' using the Transactions API. Not sure if this is related.

Any help is much appreciated!