Why did third party app notifications all revert to tri-tone, and how can I fix it? 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?
This is apparently a bug in iOS 10, based on a page I read the other day that I can't find now. For me, it's resolved every time by doing a hard reset on the device (hold Sleep + Volume Down until the Apple logo appears). I had this problem on my iPhone 6s, too, for the couple days it had iOS 10 on it before I got my 7.