Copy-only full backup - as per https://learn.microsoft.com/en-us/sql/database-engine/availability-groups/windows/active-secondaries-backup-on-secondary-replicas-always-on-availability-groups?view=sql-server-ver16
Backup Types Supported on Secondary Replicas
- BACKUP DATABASE supports only copy-only full backups of databases, files, or filegroups when it's executed on secondary replicas. Copy-only backups don't impact the log chain or clear the differential bitmap.
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hth
Marcin