I have NEVER seen the adjustment move from vibration, or from any other source, EXCEPT a 1/8" Allen wrench. That is baffling, because most of the time, you never have to change the setting . I am anxious to hear more about this condition.......I have seen the ashing arm adjustable support loosen and move....even wear completely though, but not the anthrastat setting.
Could the ashing lever stop have moved on the arm, too?. Vibration would worry me, because there is none on any machine I've worked on unless the fan was going south
and had lost a piece/segments causing the imbalance. A dry bearing box could cause the bearing surfaces to take a dirt nap, but I know you check those.