Hmmmmm this could be the infamous bug that you normally only tend to see with crossbows and archers, in that telling them to attack an enemy unit currently outside range by using single click will cause them to walk all the way up to them, rather than stop at range and shoot.
Never seen it with artillery in M2TW though.

As for your Kingdoms scenario, what was the height difference between the wall section's bottom and your bombards? If the bottom is below your bombard's height, it might be due to being out of bounds (projectiles have a certain angle bound they may fire at). But I would guess it might be someone making an error when writing up either the EDU or the projectile file for that particular scenario of Kingdoms.

[edit]
That reminds me. I think I never reported it on the buglist. Be right back