OAs are only provoked if you leave a threatened square. Creatures threaten only adjacent spaces by default, unless they have a trait called threatening reach that lets them extend that to nonadjacent squares, usually within their melee rach. The troll might have a melee 2 attack, but it should only make the opportunity attack if we move out of one of its adjacent squares. When we engaged it in melee we moved through spaces that were 2 spaces from it and it didn't attack then either, and under the assumption that regualar level 9 trolls don't have that trait, Zynna would make the decision she did.