Thanks. That may explain another issue as well, when rating 5d iv images in another program the ratings did not carry over to PM even though they were readable by photoshop, tried various ways to update the metadata in PM6 plus. The only way I was able to get PM to recognise the ratings was to reimport the images.
I am tempted to ask whether it is worth setting IPTC/XMP preferences allow for RAW files to be modified or not, presumably this has plusses and minuses, will try to find it in the help section.