Heard back from UC Davis on the Bresse pullet:
a Fractured, very soft shell egg. Egg yolk peritonitis.
I had them on layer mixed with organic chick start. Switched to just layer about a week before this happened.
They are getting better, but still getting soft shells from these and my older hatchery hens.
They also get oyster shells.
No problems with soft shells with my Marans hens. they actually eat the oyster shell. the others don't go through it as fast.
Good that it's not contagious, bad that I lost a hen to something so stupid.