it's a photoperiod? Yes, you can re-vege it. The 'auto' gene isn't active in those. they just think they are making a faster plant even though it isn't any faster.
re-vege takes forever and a day. It won't drink much but you also can't let it dry out. the leaves aren't dead, so... keep going. you don't need 20/4. you just have to avoid giving long uninterrupted darkness as always - nothing new here. Do what you did in vege phase. don't over-think it. Less than maximum might be best for now until growth returns.
if you were giving the same intensity to the plant over 20h that you gave over 12h, that would be a contributing factor - i.e. not helping but hurting.