That's right.
I tried it, 
like Sirio, "Update catalog from files" doesn't assign categories, even if "import keywords in categories" is marked
But when using "settings, catalog, add files" , the catalog will be correctly updated, and categories are OK
Method:
I have a raw file pict01.ORF (olympus raw file)
I'll use a jpeg file DSC222.JPG , having categories  animal and cat
I select DSC222.JPG and  create a XMP sidecar using "open with" function in Xnviewmp:
( I prepared this feature to use exiftool with -o %d%f.xmp  -xmp parameters)
exiftool -o %d%f.xmp  -xmp
so I get a file DSC222.XMP  containing :
Code: Select all
(other lines)
<dc:subject>
            <rdf:Bag>
               <rdf:li>animal</rdf:li>
               <rdf:li>cat</rdf:li>
            </rdf:Bag>
</dc:subject>
I rename this file pict01.xmp , 
Now pict01.ORF has a sidecar . 
in "settings, catalog" , I delete the folder containing this picture, and then  I "add" the folder.
Now, categories and keywords are correct.