I've noticed this too..it is a bit wacky.
The code that controls this is different than that which makes them trail you..so, setting following distance won't apply to this issue.
The basic problem is that bots follow directly behind you in a straight line. Once combat starts, they take a straight-line approach to the mob..stopping at the appropriate
distance from the mob based on their class and the action they are performing.
I think the only class that exhibits 'appropriate' behavior is rogue..it will adjust to to be behind the mob (but, I've seen it stand on top on before..no idea its actual coords...)
I may play with this in my downtime sessions..but, feel free to work on it if you have an idea
EDIT: This is probably a good place to start:
https://github.com/EQEmu/Server/blob.../bot.cpp#L3512