This happens also with my Samsung Galaxy Tab S2. Unless I make sure the tablet is charged to 100% before each use. The tablet sees the RC as a power source as soon as the USB cable is attached and the RC switched on.
The tablet comes up with an advisory message : This device is charging slowly, you can speed it up by connecting to an appropriate power source
Or words to that effect.........
It never charged the iPad when I had the phantom 4 and the controller lasted ages on one charge, now I have the 4 pro and the controller charges the iPad, I am having to charge the controller a lot more