Ahhhh ok sweet, yeah the Temps only went as low as 21 for the 4 hours the lights were offAre you talking about those bottom two little round leaves? Those are not new growth... those are the cotyledons which are actually the very first set of leaves. It's normal for those to yellow / die off.
Also no need to get a heater bro, those lights off temps are just fine... I usually have around a 10 degrees difference between my lights on and off temps. It would only be an issue if it's dropping below like 15 deg.
Sent from my SM-G935W8 using Tapatalk