I don't know the full details of the Eva "event", but
Fullwings said it all with respect to flight behind the drag curve.
In a previous life during test flying, the exercise was to take the aircraft as high as possible, i.e. to "nudge" coffin corner to examine aircraft charasteristics.
At that altitude, minimum drag speed was defined by Mach Number (Mmd). It was found that, at the altitude being examined, MCT JUST held M0.67 (Below Mmd) in level flight. We did encounter quite mild wind shear, and the Mach Number crept inexorably downwards, with full Takeoff thrust unable to arrest the deceleration. The only solution was to
PUSH quite assertively with about a 1200 ft altitude loss.
Flight near the Maximum Operating Altitude is for nerds. Other "fun" things I've done in testing like deliberately trying to deep stall T Tail aircraft seem like kiddy stuff compared to fiddling with Coffin Corner. Glad I don't do it any more
Regards,
Old Smokey