I got an iPhone 7 a couple of weeks ago, backed up my iPhone 6, and restored that backup onto the new 7.
Ever since then, third party apps which have non-default notification sounds, including Facebook Messenger, Outlook, and Snapchat, are all using the default tri-tone notifications.
I have restarted my phone, disabled and re-enabled notifications for the apps in question, and gone into each app to change notification settings there, if the app allowed. None of this had any effect.
Why did this happen, and what can I do to fix it?