I now know my problem. It is I don’t know enough about ICC profiles to program them…
I’d implemented a set of internal profiles, based on @Elle’s profile set. Turns out the primaries that get stored are just not the same ones I put in, so I’m missing something. So, I spent the afternoon removing them from the program, and now all works fine just using @Elle’s files.
Nothing since my return to photography has hurt my head like color management. The ICC profiles are an extremely useful abstraction, but even then you can mess up with nary a clue to guide your recovery. My younger twin granddaughter is very into color; at the age of 4 she actually chose a set of socks because they made her dress “pop”. Geesh… she’ll probably get all this way before I do… 