Yes, a ranged monster with multiple attacks will prioritize attacking the most possible targets, including its focus, over avoiding disadvantage. According to the rules:

• Page 42 specifies that when a monster performs ranged attacks on multiple targets, it moves to attack the most possible targets (including its focus) with the fewest possible disadvantaged attacks, while using the fewest possible movement points.

This means that while the monster will attempt to minimize disadvantage, its primary priority is maximizing the number of targets it can attack. If avoiding disadvantage conflicts with attacking the most targets, the monster will prioritize attacking more targets.

Summary

A ranged monster with multiple attacks prioritizes attacking the most possible targets, even if it results in some attacks being made with disadvantage. This rule is derived from Page 42.

Source: Core Rulebook (pp. 27, 41, 42, 74, 75)