The arrival of au Wallet PrePaid in Apple Pay introduces a small wrinkle in the Apple Pay Japan mix for store purchases. The prepaid card exclusively uses the QUICPay+ network. It is almost the same as the regular QUICPay network but adds support for debit and prepaid cards. It also removes the 20,000 JPY spending cap of QUICPay.
Basically you cannot use au Wallet PrePaid for gas stations, taxis, and Don Quijote. Check the full list on the QUICPay site. Look for stores with the little Q+ mark, which covers most of them.