The game doesn't properly update energy points for all fighters while it is in the background and still in memory. I noticed it shows the correct energy level if you restart the game or after playing a fight. For example, these are the energy points my gold fighters has when I resumed playing after around 40 minutes. And here are the energy points they have after I won a fight. This also affects yellow energies and not just blue ones. I'm playing on an iPhone 6s running iOS 10.3.2. But I think an issue like this affects all platforms and isn't exclusive to iOS.