It's because your temps are high, maybe your light is too powerful or too close. Maybe the humidity isn't as high as you think. I would try to bring the temps down as a raise humidity. Spray the leaves daily with water, should help.
I wait until the taproot comes out of the bottom of the rockwool before putting it in any hydroponic system. The seedling can be overwatered pretty easily and suffocate before the root appears.