I played through the first tank mission of the OA campaign without ever running into this issue, now I'm at the later one (assault the airfield etc) and I tested this in the editor as well with the same result.
I placed myself as commander of a TUSK MBT. A platoon of russian tanks is set nearby. I set "danger" and "target that tank" and my gunner begins to fire the coaxial MG at the steel monstrum. I have to either: switch to manual fire and change to sabot or change to gunner pos. and switch weapons there.
this behaviour does not change related to distance. occurs with many targets like bmps or brdms etc.
edit: first I had the zeus skill pbo enabled, now no AI mod or gameplay changing stuff is enabled - same result.
2nd edit: ok, I simply didnt know about ctrl+f to change gunner weapon. but still, this doesnt make much sense. shouldnt the gunner do that on his own? also now one of the enemy tanks fired back at me with his MG. which is of course really pointless.