Some keys on the standard iOS keyboard render wrongly

Question

I'm working on an app where, on one of the views, a few of the keys turn black, and the alpha keys turn white (instead of being dark grey and grey respectively).

The pattern seems to be:

• call becomeFirstResponder on viewWillAppear
• present a modal view
• dismiss the modal view

I have tried calling resignFirstResponder on viewWillDisappear, but there is no change.

This is happening on both iOS 8 and 9, but only on iPhones. The iPad keyboard works as expected.