It's supposed to be difficult to get a mythic drop in Diablo 4, but the latest season, Death Awakening, accidentally made it practically impossible. The Iconic Mythics are a unique treasure for experienced players, but after anecdotal research showed the drop rates are ludicrously low, Blizzard's doing some fine-tuning.
A patch is on the way that's going to respond to criticisms over season 14, according to information from the Diablo Partner Program. Specific details aren't given, but we can guess the update will fix a few known problems, such as all Mythic Uniques coming with the Crafted tag when gotten through the Horadic Cube, and the aforementioned chance of getting certain items in battle.