Hi,
Hope everything is going well.
From the official article: Microsoft Authenticator FAQs
Adding Authenticator to your new device doesn't automatically remove the app from your old device. Even deleting the app from your old device isn't enough. You must both delete the app from your old device AND tell Microsoft or your organization to forget and unregister the old device.
To remove the app from a device using a personal Microsoft account, go to the two-step verification area of your Account Security page and choose to turn off verification for your old device.
To remove the app from a device using a work or school Microsoft account, go to the two-step verification area of either your My Apps page or your organization's company portal to turn off verification for your old device.
Similar thread for your reference:
Login issue with Authenticator app due to stolen my old mobile
How to unregister authenticator on stolen iPhone?
Have a nice day!
Best regards,
Simon
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