some plants will flower with less hours of light like that.. everyone does 12 / 12 indoors simply because it's easy, and it works for every strain, but this doesn't mean that a plant will only flower if it receives 12 hours of darkness, many will flower if they receive a bit less of a dark period..
depending on how deep into flower it gets before the day light hours get longer with the beginning of summer, it may revert back to veg growth at some point.. but if it's flowering for too long before that point, it might continue to flower, even with the extended light hours..
your only option really is to find an alternative light source for it for a few hours a day to extend the light period long enough to revert it back to veg growth..