This reminds me of the early years of the Cessna Conquest, which included some fatal crashes because of design flaws that were not corrected but only patched.
Cessna eventually had to redesign the entire tail assembly and the aircraft never achieved its full market potential.
As I read the EASA document, it simply notes that one type of planetary gear configuration has lower stress levels and better reliability than the other, so it must now be used.
It conceded that 'the root cause of the failure is still not fully understood'.
Perhaps that is pragmatic air safety regulation, but it seems more like military than commercial in orientation.