It's extremely tough to say but I would check a few things with that script enabled:
1. There is a destination set for your navmeshagent
2. Your navmeshagent has a positive speed
3. If using root motion then your animations have a positive speed.
Therefore, the NavMeshAgent always stopped within 2 of the target position, not allowing the Action to trigger the arriveDistance.
This also explains why when monkeying with all sort of variables in my animations, its seemed (due to low acceleration but high speed), if a unit overshot its destination target, it would reset, and then wander again.
However, if it were able to stop, it would also stop wandering.