I can not understand why it did not happen to anyone else.
Basically, I repeated the mission 12 times always improving my strategy to prevent any rider from escaping, even to leave the borders, reaching the point where I kill them 4 instantly and verify that no rider escapes prior and triggers later or anything. It feels like it will just not allow me to complete the level, once the attack starts, regardless of if any rider escapes or not, the sentence as if they would, appear, and then it fails my quest automatically.