Arrrg I just figured out the problem: I had the PanzerCorp1-Grandcampaign installed. Once I removed it I was able to get the same results!

I'm 5 years late to the party but I'm currently doing the Tactical Puzzles and I can't figure out the Impossible Siege. I tried following the Slitherine demonstration video (https://youtu.be/iC3uP5jhMEc?si=Hvy5rr-vYHjQEb9X&t=209) and at 3:29 the enemy infantry aren't retreating for me. I tried s...