Sometimes, when I try to create an Azure Backup Vault I go though the process, and select the subscription I want to use. I wait for the deployment to go through, and then after a few minutes my vault has been created.... in the wrong subscription. Thinking perhaps I made a typo, I delete the vault and create it again. Alas, it still goes in the wrong subscription.
Not all vaults I create have this issue, but for some subscriptions, I have been stuck on this bug for days.