Unfortunately I’m using oldstyle figs as default in the font, so when AFDKO looks at my zero to synthesize the symbols, it ends up making =, >, < etc. very low (sitting on the baseline, rather than centered in the ascender height).
You could put the lining zero back in place, generate the font, open the generated font, steal the lazy symbols out of the generated OTF and put them in the original file. Then you don’t need the parameter anymore. Kind of defeats the purpose, but the result is the same.