Since 2017, setting the background property in css4j also sets background-image as well as the other longhands, so I don't think that this issue can be reproduced anymore. I'd suggest closing it.

There is one caveat though: if a layered background property is used, css4j correctly sets the longhands as well, but FOP does not support layered backgrounds. So it would not be 100% compatible with web browser behaviour.