Hello David,
I was able to test the 4 variables, {imagesupplierid}, {isid} and {imagesupplierimageid}, {isii}, in PM 5 build 17671. All 4 variables return the correct field info, on my Mavericks iMac. I will test this on the newer Mac OSes to see if I am able to recreate your problem with the Image Supplier ID variables. I have attached my sample jpeg used to identify Photo Mechanic IPTC/XMP dialog field contents. Try this file on your machine to see if the Image Supplier ID variables return : PM 5 Image Supplier ID bus#.
--Bob