I’m curious, what breed of chicks?
I have lots of theories on shipped eggs, saddled air cells, etc. Nothing I can really prove, because like you say, something “new” will still crop up. But my guess would be air from a saddled section got trapped inside with the chick, as it started to rotate, causing the bubbles, and possibly causing the death. Or I’ve seen them get somewhat tangled in the loose pieces of the membrane (from a saddle or spot where the membrane didn’t reattach to the shell) and not be able to get past it. But even non-shipped chicks have been known to quit mid-zip.
Did you happen to notice any bubbles in the air cell early on? I mean, like the air cell wasn’t just saddled, but split? I’ve gotten eggs where the air cell has bubbles around it from the shaking of the eggs during shipping. I’ve had some of those develop, but always quit before hatch time.
always an adventure with shipped eggs!

Good luck with your next group.