Somebody please 'splain to me why aircraft tasked with anti radar will drop stores and go evasive as soon as they are getting shot at. Why not put those HARMS to use?
Yep, the AI in Lockon can be a bit of a trial. If you need them to behave differently in your mission, you have to ensure your AI jet's anti-radar missile has more range than the SAM it is attacking. So that means don't choose S300 as it owns the battlefield in Lockon. Pick something less lethal. As for the defensive behaviour, that is how they are programmed, they will try to do their best to stay alive.
To be honest if it was my real skinny @ss in a real cockpit and a SAM was coming my way, I'd dump everything to ensure I was more nimble to increase my chances of dodging it. Better to live to fight another day.