some strains will convert back to veg if there is too much N, regardless of the light cycle. Same goes for plants with an N deff, they will flower on 24 hr light. It looks as if you have a sensitive strain there. Personally I would flush the hell out of it and try to rid the soil of N.
I cant see what else would cause this.... The heat could have been the stress factor over 19 days and it thought summer was here but fall light cycle??? When it got really hot here, I was freezing gallon jugs of water, ice cubes and putting them all around my plants to keep them cool. If the roots got above 90F im sure there was some damage due to heat.
reveggie buds really suk some times, I would try to thin out the weak branches, and leave the strong thick branches because those have the best chance to produce any buds. Those thin, skimpy branches will only make foxtails and not really form a thick bud. good luck.... and keep cool