It is a bit worse than the XML missing a basepath references for the media path.
Another discussion collates a series issues related to the relative media paths in Gramps.
Part of the problem seems to be related to trying to side-step cross-platform paths.
But a big issue is that importing a .gpkg simply FAILS if a folder exists in the temporary directory designated relative paths. There is no option to either skip blindly or intelligently using a compare of the checksum. (I have not looked to see if the MD5 checksums are included for the media object XML.)
I know that Betty uses the .gramps, not the .gpkg. But addresses the bigger problem should probably lead to a solution here too.
Note: another discussion has a SuperTool script to list the Constants (Envrionmental Variable) leveraged by Gramps. They can vary by session so may not be stored in the database or exported XML. But they can be useful for troubleshooting behaviors that differ between instances of Gramps.