I tried to remove the "with dissolve" transition, but the highlight animation still didn't work.
As far as I can see from the sprites, the Van character is still darkened. And the highlight animation most likely does not work just because of the previous replica of the Mark character, since I noticed that after changing sprites with pushing two characters a little further than this moment, when the Van character has a replica, the highlight stays on it for a few milliseconds and goes to the Mark character when he speaks. I think I should come up with some kind of "stub" or something like that. Well, or leave it as it is. The rest works, I don't want to break everything because of this.
In any case, thank you for your time to this problem and many thanks for this script. This is very useful for people without Python programming skills.