o/ Hello!
I was playing Stronghold today, and found that sometimes when I was trying to immobilize enemy Door Breakers and Archers, using Warrior Sword (including using the F1 burst skill) would apply the immobilize debuff, but not actually have any affect.
I would see the immobilize icon below their health bar, but they would be running away still.
I could not see any stability or other buffs on the bar that could somehow allow the enemy npc to move.
I will attempt to capture some evidence!