Hardest AI not attacking trebuchets

I built 5 trebuchets, they where not protected by my army or walls, they where on the open field attacking enemy landmark buildings, the enemy had an army and siege weapons which could easily destroy my trebuchets, but it never did, I destroyed 90% of his base with my trebuchets while the enemy army just moved around doing nothing, after a few minutes, the AI decided that attacking my walled base and defended by my army was a good strategy, well, it wasn’t, he lost and I just sent the rest of my army to finish the job.

I did some tests latter and it seems that the hardest AI completely ignore smaller groups of enemies sometimes, this is most accurate for siege engines for some reason.

I also have no mods or anything, it was a vanilla game.