Stratitis is a thing because the pole pieces are the magnets, hence why they can affect individual strings, and they have a much stronger pull than any humbucker could ever have because they're so much stronger and closer.
If the magnets in your humbucker were pulling on the strings, they'd pull on all of them evenly. That it's only affecting one string pretty much guarantees it's nothing to do with the pickup at all; I imagine the only reason lowering it is helping is because the signal is getting weaker so everything is less audible.
The first thing you need to do is change the string - it could just be a dud! Then you need to look at potential issues with the nut, saddle or a high fret somewhere.
