my plants was doing the same thing too but for the first 2 weeks of flower just like you say they would drop when the lights go out and come back up before the lights bout to come back on

this is what i think cause when it was happening to me the thing was when the light went out my 8in can fan turns off and right before the lights come on it would turn back on 30mins before also humidity was kinda high when the fan was off so it might be a humidity or air issue not sure