That's not a problem, it's a feature [emoji6]
It was actually introduced because if you watch the loss of battery during one flight, your phone actually drops faster in battery level than the RC.
Because of that the RC is charging it.
Think about the following: what can you do with your drone when your phone is out of battery? And you can't charge your phone while flying, but you can charge your RC while flying your drone, while the RC charges the phone...
This is the reason why the RC charges your phone/tablet.