Also, your code leaves "epic" and "rare" untouched, but the tooltip shows them as having been modified. Was it modified, or is that another weird artifact? If the code you show doesn't reflect the code you ran, I would suggest changing that, and also suggest leaving it "rare" and "epic" untouched ...

I was tinkering with qualities and found that some items (inserters, drills, pumpjacks) processing their stat changes for 'normal' ('unknown' too?) quality in weird way.

Simple code to update all qualities with stats from 'legendary' quality
data-updates.lua

local qualities_to_override ...