Um, maybe this was some other app, some sort of "notification enhancement"? Esp. since you've tried the backup?
But it's worth mentioning that there are two places in the app with notification settings:
- App settings -> message notifications, applies to all accounts (by default) unless:
- Optional per-account override, for this please long press on an account -> Special settings -> you'll see a few types of settings, notifications one of them, which can be set for a specific account, taking priority over "app" settings.