BuddyPress.org

The next thing I’d do is deactivate BuddyPress to see if you’re still not getting new user notifications. If you can’t do this on your live site maybe create a test install in a subdirectory. This would help isolate whether it’s a BP or WP thing. You might want to check out the WordPress forums, too – https://wordpress.org/search/new+user+notification?forums=1

Did you ever get these new user notifications? Or is it something that just started happening recently? Maybe it’s a plugin or customization issue specific to your setup.