Update: I spent 3 weeks hand-kerning my font's lowercase g and the fix was in the export settings
I kept nudging the curve in Glyphs at 1600% zoom until a friend opened my .otf on her machine and the spacing looked fine, which meant my preview was lying to me. Wasted so much time on something that wasn't even in the file, has anyone else had their editor show bad spacing that wasn't really there?