Apple Pay Suica Service Mode is a weird function that doesn’t have a counterpart on the Android Suica side. The JR East Apple Pay Suica help page mentions this. The iPhone Service Mode explanation says, “Service Mode will allow station agents and kiosks to help with any issues with your card.” The street reality is that station agents don’t need you to put the device in Service Mode, just fork it over and they can fix any Suica issue for you.
This difference exists because Osaifu Keitai smartphones (and the candy wrapper Google Pay Suica) have a dedicated FeliCa chip. Apple created it’s own custom FeliCa implementation hosted on the iPhone A Series and Apple Watch S Series SOC. But the Apple implementation did not really mature until A12 Bionic and the Express Card (Student ID)/Express Transit cards with power reserve feature. The A12 Bionic Secure Enclave supports limited NFC transactions that bypass iOS. It’s the same way a dedicated FeliCa chip works on Android.
This means that Apple Pay Suica on non-A12 devices requires iOS/watchOS to be up and running for Suica to work. Unfortunately this also means that different iOS versions sometimes have performance issues on non-A12 devices and that iOS occasionally drops the ball. Fortunately iOS 12.3 fixes all issues and has great Apple Pay Suica Express Transit performance. iOS 12.3 is a highly recommended update.
The Dead Suica Notifications/No Suica Balance Update problem happened occasionally and the way to fix it is to turn on Service Mode and leave it until it turns off automatically in 60 seconds or the screen goes dark, whichever comes first.
In this case Service Mode syncs and reconciles iOS with the Suica Stored Fare (SF) balance information from the FeliCa embedded Secure Element implemented inside the A Series/S Series Secure Enclave.
Service Mode seems pretty useless on A12 Bionic devices. I imagine it’s there more for show than actual functionality, although Service Mode is useful for cash recharge on 7-Eleven ATM machines where you have to put the device upside down to capture the ATM NFC antenna hit area.