You’re not the only one complaining about PL5 metadata handing, e.g.,:
https://forum.dxo.com/t/pl5-completely-messes-up-my-hierarchical-keywords-when-exporting
As you’ve found, in any event it’s not a good idea to use multiple programs to manage metadata.
Even with xmp syncing turned off, PL5 creates individual ‘keywords’ from sub-keywords in output image files. It shouldn’t. It’s creating additional metadata management problems for me every time I process files. At a minimum, PL5 needs an option to pass through keywords w/o change.
One thing about the current behavior is that it’s totally unnecessary. PL5 should already write keyword parts into the xmp-dc Subject tag, while also writing xmp-lr Hierarchical Keywords. Doing this should preserve hierarchical keywords while also allowing simple searches for any part/parts of the hierarchical keywords.