Added 47 seconds later
And Ilfin ran with -1 luck until the end of the game.
Added 17 minutes later
Okay, I figured out what the problem was. I tested Sigrun's attack on a level 30 and a level 20 hero... and he did reduce the speed. But only by 1 in both cases. There were Mountain Defenders and Centaurs defending the enemy. Then I decided to check what happens if Sigrun is attacked... and he didn't reduce anything. This explained everything, because my friend was defending his castle at that time.
Added 3 minutes later
By the way, when Sigrun fled, the enemy didn't lose any movement points. He just ran around on his next turn as if nothing had happened. I tested this in single-player mode.