Artillery Ballista & Focussed Ballista

Using Artillery Ballista supported by Focussed Ballista Support.
If there are enemies within regular targeting range of the ballista and the player fires an arrow, the ballista will aim at a monster instead of the location the player targeted their arrow.

This is not a bug. Focused support makes them wait for you to fire, but they don't care where you fire. If there are enemies for them to target, that's what they'll do.

This behaviour is only observable on Artillery Ballista. Every other ballista skill will target the same location as the player's arrow. Even watching the Artillery Ballista closely, it turns to face the player's target, but the falling artillery still targets the monster.

https://streamable.com/9wn9ph

Artillery Ballista specifically reads:

"Summons a ballista totem that propels a sequence of fiery arrows into the air. The arrows impact the ground in a line, each dealing area damage to enemies around it."

I don't understand how that is relevant.
Contrast with Rain of arrows
"Fires multiple arrows into the air, to land in sequence after a delay, starting at the targeted location and spreading outwards in all directions. Each arrow deals damage in an area around it.-"

Because it specifies "enemies around it" which is demonstrated in your clip.

Enemies around it refers to dealing damage around the targeted location for each arrow.