My iPhone 6+, with TouchID, switched over and made 6 digits the default when iOS made such alternative available. As soon as the iOS update concluded and as it was reactivating itself, it demanded I establish a 6 digit code. Just checked and I could degrade security and opt for a 4 digit code, but default is 6.
On my iPad 4, that predates de TouchID era, the same iOS update ended without touching the passcode at all. However, when I decided to change the code as a normal security procedure, I discovered that there was a new Options link with the possibility to go to 6. But 4 remains as the default.
As for your misbehaving iPad, it is imperative you back it up immediately while you have full access via the TouchID. You are going to have to Restore it in order to clear out the unknown passcode. A backup using the iTunes program on a computer (Mac preferred) is best since it stores all data when password-protected. See here: If you forgot the passcode for your iPhone, iPad, or iPod touch, or your device is disabled - Apple Support