I seem to remember this problem from back when I played the demo many years ago. When you raise the spot skill past a certain level, it seems like hostiles notice you from farther away. This goes for hostiles that are afraid of your superior numbers, and hostiles that are trying to catch up to you.

Has anyone else noticed this, or is it just an illusion? If it's how it works, it would suggest to me that the skill is only useful if you've either got a very fast movement speed, or have a large army AND would rather not have combat. If you have a large army you tend to want to fight, and if you have a small army you tend to want to avoid getting hit by something you can't handle, but a large spotting skill makes both harder.

Are there mods that might help fix this that anyone knows about, or is this sort of thing hard-coded?

I'd say it's an illusion. With low spot skill you never see the enemies making U-turns because they're gone before you spot them. Also, the enemies tend to run away from a longer distance than give chase. And when you have high spot, you also tend to have high quality troops making the enemies more likely to run.

Could also be your skill affects them as well, but i doubt it.

If you want to find out, there's a cheat code that reveals all enemy regardless of spot skill or distance. (For warband at least.)

It's just your feeling. There is no connection between your (or your party) spotting skill and enemy parties fleeing. Those basics cheats are for each version of M&B. (This one is CTRL+T.)