Jose Luis Angulo Rico I want to help you with this question.
Because resources are subscription-related and not tenant-related you need to consider multiple steps when you want to migrate a resource from one tenant to another.
See also: https://learn.microsoft.com/en-us/azure/role-based-access-control/transfer-subscription
Without these well-planned migration steps of subscriptions, you have only the option to move recovery services vaults between subscriptions in the same tenant.
https://learn.microsoft.com/en-us/azure/backup/backup-azure-move-recovery-services-vault
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