If the brain has too little blood flow bringing it oxygen
, it can die before the heart stops. Mismatched eyes are definitely a sign of brain problem/damage, and if the cerebral brain function fails or stops, but the medulla (brain stem) is still functioning, the heart can still be beating.
Jewish religious tradition says death comes when the heart stops, but most doctors worldwide now consider human death to be when brain death occurs. And that includes many observant Orthodox Jewish doctors, in defiance of some rabbis' pronouncements. (After all, in the 1990's (?), a dentist
lived for 112 days on an artificial heart
, while the one he was born with was removed and on a refrigerated shelf somewhere. And Orthodox Jews when putting on phylacteries pray about "the soul that is in my brain.") And hearts for transplants are taken from people whose brains have died but whose hearts are still beating well.
What you described matches the description of terminal brain damage
prior to heart stoppage.
I am sorry for your loss.