If I attack something first, Chiron will dash forward to melee range before using his Arrows. 100% of the time.
If I send Chiron to attack something before I attack it, there seems to be a 50/50 chance of it either shooting it from afar or running forward to melee range and then shooting it. I haven't yet figured out the trigger that decides which action it will take (although if line of sight is blocked, Chiron will always dash forward to melee).
If I manually click Chiron's Arrow or Shot attack, it will not dash forward until it finishes casting. If the mob is one-shot, Chiron will not move forward... if it still lives after the initial volley, Chiron will dash forward to melee range, an then use his Arrows as per normal. A good thing is... if I manually click Chiron's attack skills and it has a line of sight blocked, then Chiron will not move, attack, or dash forward... so I can try to get him into a better position to try again, without aggroing the entire room/area.