I tried it again and I think it really might be bugged. Shells actually started to land roughly where I wanted them to when I aimed a bit before the target (i.e. closer to the artillery vehicle). Then it came to my mind that maybe the wind is carrying them off course. When I checked the wind map, there was strong wind but it was actually blowing against the artillery shell's path, so if the wind was a factor it should have made them fall short, not overshoot the target.

Appears inconsistent. First time I deployed, the bear was in the water, and nailed every shot. Next time, the bear was on level gorund on an island. it was unable to land any target, practically random locations (including 20 meters in front of the unit!), when targeting units from a petrel mounted gimbal.

Additionally, when taking control of the bear, and manually firing the gun, I had no real control of the turret or barrel angle, and resulted in shots always landing nearby.

Seems like the first few shots are pretty accurate, then when you move the aim point around a lot it starts missing shots

It appears that the ground artillery inaccuracy is at least in part related to the elevation of the target. The further a target is above sea level, the further artillery rounds overshoot the target. This offset is consistent, so can be compensated for with great effort, though as the offset depends on the elevation of the point of aim, sometimes targets can be impossible to hit due to being close to a rise in the terrain opposite to the artillery unit.