Siege stopped by...?

I was playing the tutorial campaign, and had my main general sieging my 2nd target city, full of troops inside. Other than fighting face-to-face, I tried to starve them out in 5 turns.

The first 3 turns went well, and then I had my diplomat merging with the sieging army. I couldn't initiate any diplomatic talk that turn, because the old man in white was out of move. I thought: the siege will be over in 2 turns, so maybe I just forget about diplomacy and let extermination be the result.

Then strangely, I got a message box saying "this general can advance no further" next turn. I didn't look closely into the cause. However, even more strangely, the siege status disappeared! The city never falls and I wasted all turns standing there. ~:(

I doubt whether the merging of a diplomat (or other unit) will always stop a seige, or this only happens in the tutorial campaign? ~:)