Hey Jimmy5150,
A question about your nutes did you build your nutes up to the level for 12/12? Its my understanding that you should build up to the strength of your nutes for 12/12 because like if you are using GHFlora its tells you to be very careful with the measuring of the Bloom. And perhaps that may have caused it or the temperature inside of you grow box? It will go down if you dont have a heating pad or someway to keep your temp at a level that will not harm your plants! These are just questions that may have the answer to your problem in them!
I just went to 12/12 myself and my fan leaves got kinda droopy and i checked the next time i went to my darkness phase of the 12/12 and sure enough the inside temp went to 56 F and that caused it because I turned on my heater for my clonning box under my bubbler and they looked fine the next day! Good Luck