I've done some experiments with time and weather.
Made mission on same location, same date, same time. Target was M1A1.
Only variable that made big difference for me was cloud density. When it was set on 0, had no problem locking at 4.5km to the target.
When I changed density to 10, situation was same as in the training mission. For me, it locks at approximately 1.5km, but that's too close to fire.
Seems that it's not a bug, but consistent calculation for camera visibility. Just not sure that it's modeled realistically, because in the particular situation, visibility doesn't seem to