Windows 10 'Fresh Start' retains a record of the previous Recovery key in Microsoft Intune?. https://social.technet.microsoft.com/Forums/en-US/0b30ac87-bfb0-4e42-a05d-5fc73eb0090c/fresh-start-retains-a-record-of-the-previous-recovery-key-in-microsoft-intune?forum=microsoftintuneprod#1e6fcafe-8893-4002-8965-1d71a520fbf5
"For the recovery key, your understanding is correct, when doing Fresh restart, it will referencing the device, installing windows. Afterwards, when we sign in the Azure AD Account, the device will re-enroll and get the policies, apps, settings again. It pickup the BitLocker related policy. The BitLocker is enable again and a new recovery key will generate and store in Azure AD account.
Due to the action that the previous recovery key is still retained under the Azure AD account, this is the behavior in Azure AD. To double confirm if this a default behavior, we suggest to contact Azure AD support to double confirm this.