you said that this started happening when you switched to 12/12....what nutes was you feeding them before and after the light switch?
if you started feeding them bloom nutes...then i would say its a N and MG deficiency.if you start feeding them bloom nutes during the first few weeks of flowering...during the stretch...they can show sign of both of these deficiencies fast being that most bloom nutes do not have the right nutes to cover the rabid growth during the stretch.
but just judging by the pics thats a classic potassium deficiency possible due to a ph issue...k uptake drops of if your ph isnt around 6.5 when fed