If I try to enter something like Oil, glue on canvas for the XMP Artwork Physical Description tag, MetaImage breaks up the string after the comma. That is pointless – we’re not dealing with keywords here, just normal strings. Much like in the EXIF image description, where a comma is just treated as part of the string.
I guess that something similar happens with other XMP Artwork tags, too. I certainly saw it happen with XMP Artwork Title.