So, for fun, I decided just start a game and sit and watch all the various factions duke it out (using the ctrl+u and ctrl+z cheats of course). I noticed at one point one of the custom factions I had made, which had been basically made for a champion only run I once played. Needless to say, the sovereign was quite competent and was doing fairly well exploring the world by himself. I followed him for awhile, and started noticing something when he was following a released timber warg that he could quite easily handle.

1-The timber warg, which could only ever move one square at a time, would move away from him - presumable because he (the sovereign) was far stronger than it.

2-The sovereign, who could cover more tiles than the timber warg with his combined mount and tireless march, would take his turn at the same time as the timber warg. As the timber warg moved squares, so too did the sovereign, occupying the square the timber warg had previously been in.

3-Rinse and repeat. Because they both took their turns at the same time, the sovereign could never catch up to the Timber Warg, even though logically he ought to be able too. This cycle went on for a dozen turns until the AI got distracted by some unclaimed treasure.

I also noticed this happen with some darklings and a spider. Again the AI only stopped following them once he saw something more interesting, this being unclaimed treasure each time I saw it happen. Once he had decided to follow a specific enemy, any other enemies were seemingly ignored, but I imagine that just has to do with how the AI judges which enemies to go after, and that the monster he was pursuing was deemed the best monster to go after.

Unfortunately I didn't take any pictures at the time, but I'm sure it could be reproduced. It's not admittedly a game breaking bug, but it irks me to think that the AI might sometimes waste a dozen turns chasing something it can never catch.