Hi there. Thanks for sharing your findings. I had a similar issue as you and hoped that this post may have the solution but alas it was a different error that caused my particular issue. But luckily I figured out the cause for my issue as well.
In case anyone else has a similar issue of the AI move not completing its move (i.e. OnMoveCompleted() never fires), in my case I had made the mistake of overriding the OnPathFinished() function in UPathFollowingComponent, but without calling Super::OnPathFinished() first. So OnMoveCompleted() never ran.