Tom
The problem is not the AFCI, it is a grounded neutral which is a NEC violation and a safety hazard even if this fixture was not on a Arc Fault circuit, it would still be a hazard. it just would have not been detected.
As I said the AFCI did it's job, and shut the affected circuit off.
By grounding of the neutral this fixture has placed current on the circuit grounding conductor, and if you have grounding conductors connected in J boxes to other circuit grounding conductors, you now have a few circuits with current on the grounding, now if there is a voltage drop in this circuit you now have a voltage potential between the grounding and earth or another path that would equal earth, this could lead to a shock potential. if someone unknowingly were to open one of these grounding conductors, they could find themselves between live voltage at a time they would expect no voltage, and a lost neutral would go un noticed since the current would still return via the grounding conductor.