I tested around with the reveal function, and found the following.
Tanks have the MG selected as default on their main turrets. When the commander assigns the revealed enemy tank as target, Gunner does not switch weapon. Commander then gets stuck in a loop ordering to target the enemy tank until LOS is lost, or the front of the hull is turned towards the enemy. Only then (but not consistently on my end) does commander order to switch to main gun.
Location: Altis, Salt lake or VR world.
Blue: 1x Slammer, 1x Kuma
Red: 1x Varsuk, Ammo set to 0%, variable name ALPHA
Distance: 1500 meters
Positioning: Slammer and Kuma with hull side facing towards enemy, not the front
Init box: For both slammer and kuma: this reveal ALPHA
Player unit in my case is a soldier placed in the cargo space of the slammer to listen in to orders given by commander. Driver of the slammer should be able to be switched into with teamswitch.
Start mission. Commander will start giving orders to target Varsuk. Gunner will remain on MG. Commander will then order driver to move away, usually to a position where they cannot fire back effectively and where they show the enemy their rear armour.
When switching to the driver, turning the tank towards the enemy will cause the commander to order switching to the main gun, causing the tank to fire on the enemy.
This issue may complicate other issues where spotting problems are being investigated, because the vehicles may actually have seen the enemy, but have a weapon unsuitable to engage it selected until the hull is pointed towards the enemy, muddling the issue.
Sample Mission (Vanilla, Dev, Altis) here: https://drive.google.com/open?id=0B9U3xtNMOp_8NGZLekc4V1l3M00