Sans-
In this video of a tie-down run of the S-97 you can see how the pusher prop drive is de-clutched as the main rotors come up to speed. If my understanding is correct, doesn't the S-97 de-clutch the prop drive during hover? If so, I can see potential unanticipated vibration and dynamic torsional load issues in the prop drivetrain from friction clutch chatter while accelerating the prop up to speed from a stop. The dynamic torsional effects produced by friction clutch slip/chatter during engagement are very hard to accurately predict. Seems possible that the prop system failed due to operating loads higher than anticipated.
https://youtu.be/VBJZhPbMHlU