Supersession Realignment works and reasons for not working.
The reason why supersession realignment does not copy demand to successor is because there is no data on the predecessor sometimes or Inconsistency between DSO and Info Provider.
To describe supersession realignment, you must know that all demand history is stored on 2 levels:
Contains all information of every order/item line, and demand contains rather just the quantity and locations. (This is a brute simplification.) What happens in Forecast UI, is that we display content of DEMAND.
What happens in Realignment is that we copy demand from predecessor's RAWDAT to successor's DEMAND. Supersession demand is only represented on DEMAND level.
As a result it seems that realignment does not copy it, however what actually happens is realignment works fine, only the DEMAND and RAWDAT of predecessor is not in line.
This should not happen via general use, however we do not know how it was occurred.
We can monitor content of RAWDAT via transaction /SAPAPO/SPPDMRD.
You can monitor content of DEMAND via transaction /SAPAPO/SPPDMDH.
The issue is with the inconsistency between DEMAND and RAWDAT.
If you execute stocking realignment, it will synchronize them with having RAWDAT as the dominant one. (It may result in much less demand in the end.)
Once data synchronized between DSO (9ARAWDAT) and Info provider (9ADEMAND), Supersession Realignment service SPP_PDEM_SUPERSESSION_RLG copies data from Predecessor to Successor successfully.