Yeeeaah, that right there is a bunch of crap. For some reason, even if you have every character and achieved every loyalty, Mordin STILL has a slight chance of getting killed during Stage 3 of the suicide mission. The group you leave behind needs to have a high defensive rating, and if you have enough characters there alive, then you'll always succeed (your Paragon/Renegade scores to not matter in the suicide mission). Unfortunately, for some reason, Mordin still has that slight chance to die. It's annoying.

However, there IS a solution. What you needed to do (reload an earlier save if you can) was to select Mordin AS THE ESCORT for the Normandy crew after you rescue them. Whomever you send as the escort will live as long as their loyalty is secured (otherwise they'll die to let the Normandy crew live). Have Mordin be the one to go back with the crew, and you will guarantee that he lives through the game.

Glad im not the only person with this problem i didnt choose mordin for anything and he died and its really annoying i fully upgraded my ship and got mordins loyalty the only thing i didnt do was talk to him after his loyalty mission