Can someone post a savegame with this? I strongly suspect that the turrets are unable to hit those bombers because they are out of range given their shields, or something of that sort, but I can't say for sure. It would depend on the positioning of the bomber relative to the turrets, since the turrets themselves have high range -- but the mark III bomber shields are also quite high. I suspect in this specific case, this is working as intended, but without a savegame I can't be positive.