I’d use the proxy class to get the value, so I think something like this should work, but you will probably have to adjust the namespace to match your module name.
List<IMendixObject> results = Core.retrieveByPath(this.getContext(), subscription.getMendixObject(), subscriptionmodule.proxies.SubscriptionItem.MemberNames.SubscriptionItem_Subscription.toString()
Hope this helps.
Here an example I implemented:
List<IMendixObject> documents= Core.retrieveXPathQuery(getContext(),
"//DownloadArea.MediaFile[DownloadArea.MediaFile_Season = "+id+ "]",
batchSize, offset, sort);