There is a bug with the splash damage of bolt shooters. If you attack from the east-west (1), things seem fine, the target is killed and the units behind him receive some splash damage. But if you attack from the north-south (2), oh sweet Jesus...
The problem is that the calculations assume a 2D vector, but the direction is a 3D vector with the y-coordinate corresponding to height. So instead of sending the north-south direction we send the vertical direction, which is essentially zero. When we attack from the north-south, the x-coordinate is also zero, so the parallel and perpendicular distances (dot and cross products) are almost zero. That means that every unit gets hit with splash damage, and gets hit with a multiplier close to 1.
The calculation is fixed in (3). (The bolt shooter has splash radius 8m; I included Scipio's 10m radius for reference. The units are in Testudo formation.)