Incosistant display of IPTC tag in full screen mode

[calberga]

I have been using exiftool to add ReferenceNumber tags to my images, and have set the "Show text" field in Properties/Settings - Full screen / Slideshow to $D$F $X $I50, to display the values. I have a set of images (perhaps more, but I haven't searched) which exiftool shows values for ReferenceNumber in all images, but the $I50 value does not show in some of them.

What further information is needed to diagnose this problem?

[calberga]

Since no one seems to have an idea here let me posit one possible explanation. If it is false please let me know.

Can it be that there are ways of writing/reading IPTC tags which exiftool treats as valid but which IrfanView does not recognize? If this is so, is there a way to "clean up" the tags so that both programs are happy with them?

Is there anyone here who understands IPTC tag formats?