The cotyledons are dying because they have done their job. Nothing wrong there
There is a difference between dying early or dying very late....all the plants i had till now the cotyledons are always dying very late, at least 1 month into veg, even when they get covered by the upper leaf. if you have leds then you should sometimes check them out in normal white light so you can discover the deficiencies earlier because the pinky light can mask some yellowing on leafs