CSAT AI Controlled Kajman and CSAT Y-32 Variants have forgotten how to kill armored targets. They refuse to use ATGM missiles against armored targets such as MBT's. Instead they prioritise emptying their Skyfire rockets against these armored targets (meant for killing light vehicles). Only once they run out of Skyfire rocket ammo and discover they are incapable of destroying the armored target, then they may use an ATGM on the odd occasion (seems to be completely random), if they can get a lock.
In my tests, the Blufor M2A1 tanks were patrolling (so the engines were heated up) so not being able to get a heat lock does not play into the matter. I even switched to the gunner and was able to lock on targets immediately myself and kill off the remaining tanks with ATGM missiles. I further tested this by adding a to-199 AI controlled Jet into the mission which proceeded to tear apart the tanks with it's own ATGM missiles as intended.