Regarding structured keywords and the xmp sidecar file, I notice that the tag lr:hierarchicalSubject is not used by Nitro.
Searching on the web, I found the info below in a discussion regarding hierarchial/structured keywords in XMP sidecar files :
https://forum.dxo.com/t/photolab-8-and-xmp-keywords/50638
<dc:subject>
<rdf:Bag>
<rdf:li>Fruit</rdf:li>
<rdf:li>Orange</rdf:li>
<rdf:li>Satsuma</rdf:li>
</rdf:Bag>
</dc:subject>
…
<lr:hierarchicalSubject>
<rdf:Bag>
<rdf:li>Fruit</rdf:li>
<rdf:li>Fruit|Orange</rdf:li>
<rdf:li>Fruit|Orange|Satsuma</rdf:li>
</rdf:Bag>
</lr:hierarchicalSubject>The rules state that all components of a hierarchy must be written, separately, to the dc;subject tag, along with any other standalone keywords.
Then the lr:hierarchicalSubject tag should contain the full “paths” of all levels in the hierarchy, delimited by |