iPhone 6/6s Smart Battery Case Doesn't Charge Case When Phone Is Off

iPhone 6/6s Smart Battery Case Doesn't Charge Case When Phone Is Off?

I've been using the smart battery case with an iPhone 6 and it works well. When the phone is on and I charge it in the case, no problem. If I charge the case separately, no problem.

However, if I turn the phone off and charge the phone and case together (e.g. the phone is in the case but turned off), the next day the phone is 100% and the case is at 0%. Does the case not charge if the phone is in the case and turned off?