Seedvault is not a good backup solution. The devs who promote it don't have a lot of credibility. It basically doesn't work half the time and they know it, but pretend it's great to pad their resumes.
I think it was back on LineageOS 16 that it was actually broken for the entire release. It wasn't even possible to do a basic backup at all.
Google for "site:reddit.com does seedvault even work" for dozens and dozens of examples.