The immediate thought would be to breed Swift Foxen until you get one with Swift 2 to release, but is that how traits work? Can they combine and level up?For now, there is no explanation in the game. However, there is a certain logic. Swift is an inherent trait of the foxen, for example. Instead of a bug, it's more like the game isn't finished. Swift 2 is necessary to level up Foxen Essence 3, but Swift 2 only appears on foxen with at least Essence 3... Do you see the problem?