Magisk Mount Namespace
On recent versions of Android, it is necessary to use a global mount namespace in magisk for root apps to access /data and /data/data folder content. Otherwise you'll only see the app's data folder itself.
It will allow 3C App Manager and 3C All In One Toolbox to backup app's data as on previous versions of Android.
PS: It would be nice to ask Magisk's developers to make it the default ;)