Thanks for your insight and sorry for the delayed reply. Had to put this down for a while.
I'll keep those possible issues in mind.
In this case I don't think it's either of those though.
The first time I noticed this was with a physical luminaire but when I was trying to isolate the issue I created a family solely with the light source / IES file without any body, not nested etc. and still same problem. So fairly convinced it's something to do with the IES file itself.
Still none the wiser on this one unfortunately.
If anyone has come across this before would be very happy to have your comments.