dc32f0cfe84def651e0e There are known issues with Seedvault, especially for secondary user profiles. It does not seem to be reliable.
It will eventually be replaced with a new backup solution developed by GrapheneOS but it's not high priority at the moment.
In the meantime, I suggest you create the backup locally and then copy it manually to your USB drive.
A word of warning: Do not rely on Seedvault backups. Always backup your data manually separate from Seedvault. Seedvault has been known to break. Even if you manage to backup your data, it doesn't mean you'll be able to restore it.