Why is episode 12 of Kekkai Sensen severely delayed?
The current reason listed on the show's website (link in Japanese) is that the final episode is planned to be longer than the 30 minute time slot it was originally allotted. The production committee has postponed the airing of the 12th episode until further notice (perhaps as they attempt to secure a longer time slot), when its Twitter and official pages will be updated with the decided date.