As far as I remember,
if xmp.lr.hierarchical subject is present in xmp file, dt imports them and ignores xmp.dc.subject (single tags).
If xmp.lr.hierarchical subject is not present it imports the single tags
But it imports also itpc.keywords …
There is no way to control the dt behavior here.
To make it work I would suggest to clean up your xmp files from any xmp.dc.subject and itpc.keywords , using exiv2 or exiftool and make sure the xmp.lr.hierarchical subject is present.
Then, deleting the useless tags is fast even if one has to do it one per one.