Plastic has been deformed over the years - you need a steel ring inside to reinstate the shape. Tightening down onto unsupported plastic is bound to cause it, I see it on a lot of heaters where the air intake pipe is plastic which appears to be rigid but over time becomes tapered when tightened down on.
It seems keeping the clamping force of the clip closer to the AFM helps solve the problem.
lots of replies on FB saying the same so a common problem it seems
an air leak here I think would cause it to be lean which is not good.