The stem clamp cracked in 3+ places; I had checked torque but not "torque sequence"
Bolts are SRP. Perhaps I overtorqued?
I think it's a clear design flaw:
you can see both cracks starting on the inside of the front plate.
The edges are way too sharp and thin, should be more rounded than they don't start to crack so easily.
Florian is right: I took the stem in to LBS yesterday, and they mentioned that the clamp on the older Deda Newton was a "semi-recall" (i.e. unpublicized) item, because of many similar cracking issues, due to flawed design.
The clamp has apparently been redesigned, and your LBS will replace the clamp for free.