I also notice that the background image and Applications folder alias that are supposed to be in the disk image don?t seem to work in Catalina, so I may give up on having jpackage create my disk images as well, and do that using native tools, until that can be fixed. (I realize it is in early access still, and hope these issues are being worked on.)
What exactly is wrong with the background image? Were there any build errors reported during packaging?
The creation of the disk image is done via some script and you can tell jpackage via the option --temp <some empty dir> to dump that into a temporary folder. You can modify it and place it into another folder which you tell jpackage via --resource-dir <some dir with the modified script> Maybe you can fix your issue that way or at least you can find out what the problem is.
I am still on the previous version of macOS until all these Catalina issues are sorted out.
Java/JavaFX on Catalina is a mess right now.
