6 digits "old passcode" required during Apple ID setting
After I updated iOS in my iPhone 6, I was asked to do 'Update Apple ID Settings'.
In the process of this update, 6 digits "Old Passcode" is requested after I successfully input Apple ID password and current passcode. My current passcode consists of 4 chars + 4 digits which is requested by my company policy. I only used 4 digits passcode before my current passcode.
There is no way to remember 6 digits passcode which I never used. Is this any kind of bugs?
iPhone 6, iOS 11.2