Please open a TAC case for tracking. I experienced the same, and found out that QuickConnect cannot delete the WLAN profile in Android 6.0, and that is supposed to be changed by Google 'by design'. An App may only delete or modify WLAN profiles that it created itself. Engineering is aware of this and working on it. Please send send me a personal message with the TAC case number if you want me to have a look at your TAC case.
As a workaround for now, you can Onboard over another SSID (Guest SSID may work), and delete the SSID that you are Onboarding to (typically the Corporate SSID) before Onboarding. In my own testing, I succesfully onboarded two Nexus devices running Android 6.0.
Herman