OK, I'm hearing the same thing from the V8 guys. If the head is softer than about 65 Brinell, the head bolt tension won't stay after torquing.

And that's one way to tell if the head is soft: if the head bolt washer has sunk into the metal suface.

While engine overheating won't soften the head, continuing to run the engine after the coolant boils away will bake it, allowing the head temp to rise above 250C and start softening the head. They said when it's soft around the exhaust ports, that's probably the cause, rather than overheating while machining the head (they heat the head to straighten it oldschool, or to fit guides or seats).