Hi there,
It seems you have not set the -VaultId parameter correctly to get the expected result.
A container that is registered to an Azure Recovery Services vault can have one or more items that can be protected. For Azure virtual machines, there can be only one backup item in the virtual machine container. Set the vault context by using the -VaultId parameter.
You can read more about this from the below article
Get-AzRecoveryServicesBackupItem https://learn.microsoft.com/en-us/powershell/module/az.recoveryservices/get-azrecoveryservicesbackupitem?view=azps-7.5.0
Azure Backup PowerShell samples https://learn.microsoft.com/en-us/azure/backup/powershell-backup-samples
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