maybe the sun was too bright for them all day right away, i’ve heard of people hardening them off by starting with a couple hours each day or something, or maybe 20" is too close (ive never used MH). it does look like light bleaching i’ve had before after i sprayed method 1 with the lights on, have you been foliar feeding?
also maybe pest dmg, something eating the chlorophyll? be careful about bringing plants inside and out, it’s probably how i brought thrips in to my indoor garden.