Assuming that Bahamut is stronger than the twelve, because the sealing using the 12 failed is a sign of faulty logic. Assigning the failure to the twelve is impossible, as we do not know to what degree they were actually involved or not. We don't even know what the twelve really are, just what Square Enix calls them. Also that little ritual also involved the archons and you could also assign blame to them. Or we all just didn't pray enough, either way... what really happened most likely was... Square Enix said to themselves, "Would story be cooler if Bahamut was resealed and everything up to this point was pointless? No. Okay, let's do this crazy sealing ritual but for whatever reason, flashy lights and what not, it will fail. Why does it fail? I dunno, it just does. Allright then where does Bahamut go after? We got this cool dungeon that we made but no final boss yet. What's Bahamut doing there? I dunno, he's sleepy? Okay."