Rocket hit but did no damage

I am in a Clash of Nations round as the United Kingdom. I have taken over France, Sweden and Germany, and am pushing into Poland, who has taken C. Russia and most of Caucasus and Ukraine. I have taken Danzig and the province west of Warsaw, his capital. I fired a Level 2 Rocket Poland's TD stack in Warsaw, which he was trying to retreat eastward. That stack had full HP, but it did not have enough AA power to kill the rocket before impact. When the rocket hit, I knew it hit because a Level 3 Tank Plant in Warsaw was down to Level 2, and I saw the "stain" of the rocket impact on the map. However, the TD stack (which was still in view range) had not taken any damage at all. It was still at full HP. The rocket does a maximum of 22 damage to heavy armor, but it would have probably done less because the TD stack has some AA defense. But it seems to be a bug that it didn't take any damage at all.