I currently have an application in external testing via TestFlight. I'm seeing an odd, repeatable behavior: IAP fails when using Touch ID to authenticate. The dialog to confirm the purchase appears, and as soon as I authenticate with my fingerprint the window disappears and no further action occurs. If I attempt the IAP again but enter a password, the transaction completes as expected.
This occurs on my iPhone 6 running iOS 9.2.1 release, and also occurs on an iPhone 6s plus running iOS 9.3 b3. I can't for the life of me think what would possibly cause this to fail, is there something with TestFlight that may cause errors with Touch ID IAP authentication?