This happened during the same match on Hill and Dale map. Both the hard and hardest AI made weird choices with building placements when a landmark was involved. The Hard French AI built a market next to its already built Chamber of Commerce and it wasn’t meant for training traders either, as the AI only started training those much later.
On the opposite team, the Hardest English AI started building a town centre and seconds later placed down the foundation of King’s Palace right next to it and completed both.
These issues seemed similar to me, but what’s causing them may be different, since the English one happened just as age up became available.
In another game that I set up (dark forest) just to spectate the AI, the hardest Rus AI built a market next to its Golden Gate landmark. Again, this wasn’t in order to start producing trade but instead balance its economy, and indeed it made multiple resource exchanges right after construction as if the Golden Gate didn’t even exist. In fact the AI seems to completely ignore the Golden Gate trade mechanic the whole game and never uses its built up supplies. This applies to the hard AI as well. The hard AI built an unnecessary market close to its Golden Gate too, but there was a monastery between them.
The hard AI also built a High Trade House right next to its regular hunting cabin that it finished building moments before this.
The Rus landmarks seem to be even bigger offenders since the favourable trade is completely ignored by the AI and instead it uses the regular market for that. Also if I’m not mistaken the hunting cabin and the landmark equivalent would cancel out each other’s gold generating aura (no trees here though).
Edit: As a sidenote referring to the original post: The French AI with their market issue used Chamber of Commerce to buy or sell resources both before and after they built the extra market. The English used both the landmark and the town centre to produce villagers simultaneously.
Rus AI behaviour pertaining to trade and hunting cabin landmarks confirmed to both happen with hard and hardest difficulties. The market is even rebuilt on the same spot if it’s destroyed.
Thanks @LamppuKioski! We appreciate this insight. I’ve logged this internally so the team can track this behavior.