The spindle fracture tells a big story.
This was not a repeat of the planetary failure chain of events, as I mentioned earlier, blades stay attached even after powered impact. Look at the tail rotor broom sticks, still attached. The separation is in a completely different spot, not at the conical housing, with struts and all.. No, I would strongly suspect that this started with the blade attachment shown broken in the photo.
fracture surface, even at the view shown, is not a clean shear, it has some jagged features on the aft side, and the area near it is clean from paint.
Then, there is an inward crushed area below and forward. This could be the indication of blade twist overload of the remaining segment of metal.
In this theory, crack exists, and is not detected, rigidity holds the geometry. Initial collective load is a "push" on push rod, blade twists, and crack opens further, BTM (Blade Twisting Moment). Damper loads contribute, possible to be complicit in root cause.
Airspeed increases, transitional lift allows collective to drop. Push rod "pull" begins local crushing, blade now unstable and flutters, before separating from rotor.
main shaft fractures from massive overload.