Hi Eva,
we are facing the same issue and created a SAP incident for it. SAP responded that this is just a display error and the correct write-back system is used when saving plan data. I don't know if the planning actually works for you after you've promoted your workbook?
For us it was more than just a display error, we couldn't save any plan data after promoting a workbook and had to manually refresh the datasources, change the write-back system, save the workbook, close and re-open the workbook and only then it would work.
I explained this to SAP but they were not able to reproduce this issue on their systems.
However, I did find we were doing one thing differently from SAP: we are using OLAP connections that connect directly to the planning Bex query whereas SAP uses an OLAP connection that connects just to the BW system and then select a planning query only when creating the Analysis Workbook.
I tried this approach as well and it works now! The system for write-back parameter is still showing our DEV system after promoting to TEST, but entering and saving plan data in our TEST system works correctly without any manual change.
Hope this works for you as well,
Best regards,
Joost