MH have been pretty inept on this front, I agree. Instead of dealing with refunds in a proper, and timely fashion, they played a game where they forced people (who qualified for a refund under their own Ts&Cs, hoping that people will settle for vouchers).

I have my own saga with MH for flights affected by COVID19, and like OP I just asked AMEX for chargeback. It came with the usual caution that MH could come back to prove that they have the option to not refund me, but I have their Ts&Cs in writing. So far 4 weeks after the chargeback with funds in my card, no issues so far.

MH will seriously lose out as they have no visibility of card companies doing chargeback, and losing loyal customer base too. Double whammy.

Such short sighted leadership decisions.