TL;DR: Your luck stat is not bugged, just add luck on your hireling.

And here is luck formula when not using a hireling:

**(Affixes + Natures + Fortune + Difficulty) + Fortunate**

Where:

Affixes - sum of Luck affixes on gear and pet

Natures - sum of Luck natures * 25

Fortune - hero points spent in Fortune * 2,5

Difficulty - number of difficulty * 50 (Very easy is number 0, Mythic 3 is number 8)

Fortunate - perk (200%)

(Affixes + Natures + Fortune + Difficulty) is capped at 650% or 812,5% if you have Epiphany(5) set.

All luck is capped at 850 or 1012% with Epiphany(5).

If your max luck was 850% then you weren’t using Epiphany, but you have Fortunate ascension perk. If your luck went from 700 to 720 by adding 40% then that means you are using a hireling, because the luck you added was divided by 2 (look at the formula when using a hireling).

If you add some luck onto gear and it does not change your Luck for Drops in Adventure Stats page, then check All Stats page 8, because you have probably capped your luck for your hero which is 650%. If your luck is not 850% but 805% and you have capped one hero luck at 650% then your other hero is not capped and has 460% luck in All Stats page 8, because (460 + 650) / 2 + 200 + 50 = 805. The solution is to raise your other hero’s luck by adding luck affixes, luck natures, fortune hero points or changing the difficulty. Difficulty calculated for a hireling is the last difficulty your hireling played on as a main character. Your hireling needs only 550% luck, because (550 + 650) / 2 + 200 + 50 = 850, which is a cap for luck without Epiphany set on main character. 550% luck can be easily achieved by having the last difficulty your hireling played on be Mythic 3 + having 20 hero points in Fortune + having all 6 gears natures of Luck, because 350 + 50 + 6*25 = 550.