AI issues creating a scenario

First of all, I made this scenario, years ago, using my CD version of AoE2 Conquerors: A kind of Urban Battle scenario. I'm trying to reproduce the scenario on my Steam HD version, but I'm running into some AI issues.

The scenario has a massive city in the middle, controlled by a passive AI that just exists. On the four sides of the city there are small, open settlements for each player, human or AI. They're surrounded by a basic amount of resources and separated by natural barriers. The middle city becomes the most convenient way to reach your enemy, creating interesting scenarios where your archers hide behind houses and calvary has to run through paved streets, all while houses are on fire caused by collateral damage by siege engines.

A more advanced version of the scenario gave the players a time limit. As the game progesses, the middle city suddenly closes its doors to the players' military (becomes neutral) and at some point it turns aggresive towards all players, turning the game into a sudden death mode.

Now to do this I only need one simple thing: The AI must honor the starting diplomacy and the triggered diplomacy changes.

It does not. I swear it did before.

Right now, the middle city turns aggro almost immediately (towards human players mainly). Or it resigns right at the start. Triggered diplomacy changes are respected for five seconds before turning back to normal. And locking diplomacy locks triggered diplomacy changes as well. Locking diplomacy serves only for the basic version of the scenario, with a passive central city, which is fine, but...

Any help?