I'm not 100% sure but
AFAIK the Thielert engine simply cannot run without a FADEC. Injection timing comes to mind. So defaulting to "feather" if both FADECs fail sounds sensible to me.
Main question is why the power supply is not sufficiently robust/reliable to ensure power to at least one FADEC in case of a near-flat (main) battery combined with a transient electric load that exceeds the power output capacity of the two alternators.