This issue was inherited from the original AOE3.
When moving a group of units near enemy units/buildings, our units will automatically attack the enemy except for artillery units.
Artillery units will remain idle in Limber Mode.
They will switch automatically to Bombard Mode only when they are being attacked.
Some may say that it is up to the player to switch his idle cannons to Bombard mode.
However, there is a problem with that when we have different types of artillery units in our group.
When we switch to Bombard Mode a group of artillery units, only ONE type of artillery will be switched to Bombard mode.
The other types of artillery units will remain in Limber Mode.
This means we now have to select each type of artillery units separately in order to switch them to Bombard mode.
Solution: either switch all artillery units automatically to Bombard mode when they are idle and near enemy units/buildings.
When we switch a group of different types of artillery units from Limber Mode to Bombard Mode (and vice versa), apply that action to ALL the different types of artillery units that are in that group.
Personally, I think that idle artillery units should switch automatically to Bombard mode when they are idle and near enemy units.