Hi Mario,
thanks for explaining a few things; I have progressed a little bit further, but I'm still not there.
Attached are screenshots of my metadata-template and the result after I applied that template to a picture of myself and my wife Barbara, did a write back. The IPTC keywords seem to be OK (Trefwoord is Dutch for Keyword). As you see: there is no XMP keyword.
Which exact tags did you use to get the above shown result?
How are XMP keywords filled, what to check/uncheck?