Hi,
I remembered that I hadn't included my build number (Photo Mechanic Version 5.0, build 19395 (69a3200) and the fact that I had "prefer IPTC/IIM" set, and I decided to look at one more test file before I updated my post. Strangeness happened.
I ended up with test files written by Photo Mechanic, Photoshop, ExifTool, XnView, and On1 RAW.
All of them except the ON1 RAW file read fine in PM.
The ON1 RAW (Which turns out to be the one I sent earlier) file reads fine in all the other programs, but doesn't in Photo Mechanic.
In ExifTool, that tag looks identical in all the files, from all the programs.
But in a hex editor, the header of the ON1 RAW file looks quite different.
So, I (sort of) can't reproduce my own bug.
But it is puzzling that all the other programs can read this particular bit of XMP and Photo Mechanic can't. Obviously, there's something here that I'm not understanding correctly. Photo Mechanic is apparently less permissive somehow?
-Carl