your gone for a week ?
so someone else is doing the 12 / 12 's ( also, with you )...... ? if you are doing 12's outside ( total darkness like tent ) you would not have issues they would act as indoors do under 12's ...... it's not the sun. the sun is providing much better spectrum of light by far than gravita's.....
so your tarps are not cutting off the sun because of some issue. can you show pic's of outside the greenhouse and how your pulling those tarps. you must put them on 12's at the same time every day and pull them in a.m. at the same time every single day ( 70 days in a row .... ) , if you or your partner missed a day or pulled tarps 3 hours late would stunt the flowering process...... is your black tarp thick enough ? pull the tarp over at noon and check it then for light leaks or you will see the tarp allowing some sun because its not thick enough.
and how are you pulling fresh air into that greenhouse ( during sun and during dark period with tarps ) ? how many fans inside.... ? and those lights up top should never be used. your location should be getting 8-10 hours of sun then the a.m, p.m., dawn - dusk lighting as well. when did you start them outdoor from seed or clone ? what month ?
and how did you run those lights up top of greenhouse ? ( month / latitude / compensation time )
narrow everything down until you find the stress issue .....