Hi @sakuraime , thank you for reaching out to us!
A database that you add to an autoprotected instance might not immediately appear under protected items. This is because the discovery typically runs every 8 hours. However, you can discover and protect new databases immediately if you manually run a discovery by selecting Rediscover DBs.
Refer to the FAQ page - https://video2.skills-academy.com/en-us/azure/backup/faq-backup-sql-server#why-cant-i-see-an-added-database-for-an-autoprotected-instance
Also, you can comment on this feedback item - https://feedback.azure.com/forums/258995-azure-backup/suggestions/37176106-autoprotect-does-not-work-for-me-when-i-add-a-new and up-vote the request.
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