Defy II's Damage Being lucky not working?

I'm using Constricted Viper together with fragmentation rounds. I tested out my damage againts bosses and the luck feels the same. Has anyone else tested this out?

you can calculate the expected dmg increase according to the formula here in the wiki.

For most dmg types the "luck" modifier does not increase dmg by much, as min and max dmg are closer together compared to e.g. lightning, where luck helps the most.

Yeah I know but still. I placed in heft for testing, just to have the biggest variation possible on regular crossbow attack. After I crunched the real numbers in-game there was no difference. But maybe I did my calculation wrong or got very unlucky (doubt because I did bursts of 7 shots, 7 times, so the tallied up damage should have been higher)