Slitherine

In the above screenshot, I have a Roman force under siege. The unit is composed of 8 units/14 power. The Celticii are besieging with 12 units/56 power. Neither have moved this turn.

According to the manual, The defender in a battle is the side with the biggest number of units not having moved this impulse (the only exception to this is if the defenders have sortied
from a besieged city). In this case with neither the Celticii or the besieged defenders having moved this turn, the side with the biggest number of unmoved units is the Celticii. My outside Roman army, 15 units/119 power then moved into the region and attacked the Celticii. So the defender should be the Celticii and my outside army moving into the region should be the attacker. However in the screenshot, note the attacker is listed as the Celticii and my outside army is listed as the defender. In the ensuing battle, that is how the forces fought with the Celticii receiving the attacker bonus.

Unless, I am misinterpreting the manual, it appears that the outside Roman army should have been designated the attacking force and received the attacker bonuses. Is this a bug?

I'll leave it to the devs to comment on your questions regarding the potential bug.

However, I think that in the assault of a town the units inside the city should always be the defender irrespective of the size of the units. Obviously, if it is a sortie from the city by the defenders against an army that is besieging then the units inside the city should be the attacking force.