Ive gotten that luck, but I did get a faint corruption aura, so not complaining.

I've farming normal Bandit King for his crown, i farming until use a gear soda and challenge power.

(45% (12.5x2%+20%) Chance, but i not sure will stack be like 65%)

More like 4.5% and 6.5%,because even with that high chance of getting something,it will then pick which item will be given,and guess what? It will never give you the item you need or want.