I don't know if anyone has experienced this, but it's a (¿bug?) that is there since the beginning of time (almost since 1.08 version).
Playing coops in the west side versus SLA, if you disable a UAZ killing the crew and use it, the enemy doesn't fire at you (only in very rare occassions). It is even more noticeable at night. Maybe BIS tried to simulate the IA thinking that an UAZ is not a threat at first, but seems that they take their time until they discover it is killing and driving over all his army.