NEVER and I repeat NEVER send an IDML file to a printer.
Send a PDF.
I understand there can be issues but this I did not expect. Thanks for the advice. I won't be packaging files to include an idml file anymore!
My guess is the “printer” is opening the file in CS6 on an El Capitan machine.
Just a guess mind you, but any printer that can’t afford $50/month to service clients properly should be avoided at all cost.
Actually, my files go to the prepress department at the publisher and I think that's who is opening and distilling the pdf for the printer. I need to find out when they are upgrading to CC2015 and, until then, I guess I need to revert back to using IDCC2014 and check the files over carefully to make sure they are OK.