Two decades years of certifying 777 electrical systems and I have never seen anything like this before.
Having read the limited public informational available (particularly the AHM report), and just thinking aloud, I can't help thinking...
The 777 has only a single Back Up Converter controlling both Back Up Generators and both Transfer Busses.
..and wondering...
Would this incident still have happened if each Back Up Generator had it's own, completely independent, Converter?
I am not speculating on the cause - I just don't have access to the detailed information to answer this question.