Hayo,
I contacted HDRSoft by email last Wednesday but haven't heard back.
I can tell you that the XMP in the TIFF file only has their proprietary "Tone Mapping" settings (Method, Brightness, Compression, Contrast, WhiteClip, BlackClip, ColorTemperature, Saturation) and no IPTC4XMP metadata. However this is stored in the old IPTC IIM record. So if you were to load IPTC first in your PM IPTC/XMP preferences for these files then it finds the IPTC data, and PM was able to update it.
But really they should correctly write the tags in increasing numerical order, add a package wrapper to the XMP, and put the IPTC data in the XMP along with their proprietary settings.
HTH...
--dennis