@h_d Affinity Photo does support the IPTC Core fields, both in their original IIM guise and when given in the XMP IptcCore namespace. Per the standard, IIM Caption/Abstract should be turned into XMP IptcCore.Description, then can become XMP dc:description for export. (The Windows "Properties" dialog is capable of populating from the latter; it amalgamates from a few different data sources for that dialog.) XMP is used internally (mostly).
It looks like the IIM->XMP step may not be happening properly here. ExifTool also does not seem to perform this step when it emits XMP; this surprises me too.
@BillCar Thanks for your report; I'll take a look at whether we can do a bit better with the output of Picasa.
N.B. The submitted image file has no Camera maker/model metadata, nor tags or keywords. While there are some naming/terminology differences to some other software, this is somewhat unavoidable with all the variations out there in the world of metadata, and we've tried to find a balance between common sense and following the standards there. Naming differences don't by-and-large affect whether we can import/export metadata fields though.