I have a fluorescent light plugged into and outlet that comes BEFORE a GFCI
outlet on a circuit.
About 5% of the time I turn the light off (I have a switch leg going to that
outlet) the GFCI trips.
It only happens when I have an air filter turned on that is plugged into the
GFCI.

It is not clear to me why turning off the light would affect the GFCI coming
after it. (Yes, I do have the GFCI installed correctly; not backwards.)
Even if there was a ground fault, and I can't see why there should be one,
the GFCI wouldn't see it.

Any ideas?