Once fighting at close quarters is initiated, the gameplay ist broken down into a cycle of rounds[...]
CRB 94
........ While the close combat is more tactical (selecting stance and actions that might be best in this situation), during the Opening Volleys you only can shoot and approach. So the players can't react to the threat of an adversary with Might > 1. During close combat they can.
And I think it fits the narrative of such a fight pretty good: the Opening Volleys are a coordinated effort of each side to let a rain of arrows go down on the enemy. You are pressing foward and shoot until you have reached your enemy. Then the real fight breaks out (and the Rounds begin).
The rules on Might are on page 143. This is specific in stating that a Mighty Adversary attacks 2 (or more) times per round in Combat.
It is both acceptable and reasonable from RAW that during Combat both sides may have archers firing from Rearward. Both sides may be firing at the other side's archers naturally.
Consider a hypothetical Archer-Troll with Might 2. This adversary fires twice per round of combat, while the player-heroes get one shot per round of combat.
Logically this is an exact match for both sides shooting in the Opening Volley. In fact the only thing that is different is that the fighters who are running towards each other don't get a round of Close Combat.
This is why it is perfectly reasonable to have both interpretations . RAW does not DEFINE an answer to the OP, so we should come to our own decisions. Both are possible.
Edit: As a player, I would of course follow what the LM had decided on this issue. But, if I were the LM I need to have designed my own Archer-Troll in advance, and made up my mind about its firing rate. (My Archer-Troll would shoot twice in the Opening Volley, just to get the reaction from the players! )