Hi,
I don't think where you create this field via AET has any impact on whether the field gets persisted or not. Technically it should get persisted.
You need to be careful about the inmap and outmap, they should be populated correctly.
You might want to try what's suggested in this thread for your issue, it seems quite similar:
https://scn.sap.com/thread/1308848
Regards,
Parul