Subscription is the one of few objects that doesn't work with like to like API field name copying in Payments2Us.
However, with a simple flow on the Payment Txn (status equals Matching Complete - so it will not impact any payments and subscription not equal to null) that can then update the subscription record with the relevant field (e.g. Campaign).
NOTE: Flows and customisations are NOT included as part of our standard support. For any further details or information, you will require our Premium Support service.