When on the fighter selection screen, if you directly open up a character’s skill tree by clicking the level in the corner of their card, then go back, the background will be gone. This happens in all modes other than Training and is cosmetic only. It doesn’t look like this during the fight itself. Shown in a prize fight, daily and story mode here: It’s consistently happened from directly opening the skill tree from the fighter selection screen for me (never clicking the card and opening it from the character’s screen). Phone is an iPhone 7, running iOS 11.1.2.