Anyone else have issues with the AI just slowly walking at them in battle?

This doesn't happen all the time and I've yet to figure out what triggers it, but I'm often having issues where the AI will decide to advance on me at walking speed, presumably to save vigor. Which is fine... except it keeps doing it even when I'm bombarding it with spells or artillery, sometimes letting me easily wipe half the army before they even reach me.

I can't recall if this is a vanilla problem, but I don't THINK any of my mods should be messing with the battle AI. Does anyone else have this issue, and/or know of a mod that fixes it?