They are probably using threat levels, too to pick targets. So an Adder having a fit will not register when there are bigger targets to shoot or something is shooting them. I think it's the same function that makes them not suddenly target you from everywhere when you happen to shoot at a clean ship and get an assault bounty. At least not when you're not jumping to the top of their priority list.
The only time the cops ignore my target is when they are already engaged on someone else.