Stress fatigue and fractures doesn't show itself after one dive without completely tearing down the craft and inspecting components - and this was probably the deepest dive they've been on.

pressure increases on a log scale the deeper you go, so you need to account for that, evidently, they did not account for it, and they also failed to understand requirements for regular teardowns and inspections of prototypes.

Honestly I don't find it surprising at all.

There are multiple times where people have died due to fractures as a stress fatigue in different areas.

I remember a story about the crash of United Airlines flight 232, in where a DC-10 suffering an undetected stress fracture after many flights finally broke an engine to the point of it severing out all hydraulic lines to the control surfaces - they had to try and land the plane via throttle. It's actually a very interesting story if you want to look it up - they even made a movie for it.

The issue here is the number of dives it took before something failed catastrophicly - you usually engineer it to withstand the stress for X number of dives - 6 dives are far too few and and it is indicative of poor design and poor maintenance. - compare UA232 where it only happened after multiple years and 200+flights before it finally failed catastrophicly.