are the records present in BMC.CORE:BMC_UserCommunity class in both Staging and golden dataset?
The ast:usercommunity form is a join between AST:Attribute and BMC.CORE:BMC_UserCommunity form.
The link is based on the condition --> ReconId = ReconId or ReconID = InstanceId <-- (verify the join condition)
We need to check if the recon ID generated is present on BMC CORE and AST Form or not?
Is the record present on the regular form as well i.e. "BMC.CORE:BMC_UserCommunity_"
Have you tried to create a new record and reconcile it to check if that is visible?
also please read about abstract classes : BMC Atrium CMDB data storage methods - BMC Atrium Core 8.0 - BMC Documentation
Superclass of UserCommunity is collection class which is in turn an abstract class.