Hi Cleo,
Could it be that something went wrong with the database migration or release upgrade, e.g. an incomplete copy action? Did you discuss the issue with the Basis Team?
Did you try to delete the initialization and all data stored for the affected data flow, initialize the delta once again and load the PSA (as a work-around)?
I suggest to move the discussion to SAP BW Powered by SAP HANA. Please provide more details re. SP level old system and new system, does it affect all DataSources or only few, etc.
I also advice you to create an SAP Support Message.
Best regards,
Sander