When a monster's movement path is ambiguous—meaning there are multiple equally valid options according to the movement priorities—the party decides which path the monster takes.

This is supported by the following rules:

• If the monster could move to multiple hexes that maximize the movement priorities (such as attacking the most targets, triggering the fewest negative hexes, and using the fewest movement points), it moves to the hex that requires the fewest movement points.
• If movement is still ambiguous after applying all priorities, the party decides which hex the monster moves to.

Source: page 75

This is further confirmed on page 40, which states:
"Sometimes the rules in this section might not fully clear up ambiguity regarding how a monster should act. In such cases, the party gets to decide the ambiguous choices. This decision does not have to result in what is best for the monster."

Summary:
When monster movement is ambiguous after applying all movement priorities, the party decides which path or hex the monster uses.
References: pages 75, 40

Source: Core Rulebook (pp. 40, 41, 42, 74, 75)