@hutchnd Sync code has been 25 words for a while now. So with you doing 24 words, means yours is outdated. On top of that, current 25 word code actually changes daily, so you can’t keep it as a backup.
Good news is just the 25th word is what changes, at least for now. So typically for someone like you, can just add the 25th word. One of the devs created a site where you can check the 25th word, which you just need to add to the end of your phrase. Link to site is https://alexeybarabash.github.io/25th-brave-sync-word/
Todays’ word, if you want to hurry and add it, is avoid
As to sync, keep in mind their warning.
Warning: Brave does not officially support using the Sync code as a backup and you should not rely on this continuing to work in to the future. Use the export functionality in bookmarks and the password manager instead.