See Lawrence Abrams' 16-Aug-2022 Windows KB5012170 Update Causing BitLocker Recovery Screens, Boot Issues on BleepingComputer.com
As of 19-Aug-2022 Microsoft has officially confirmed an issue where some devices might start up into BitLocker Recovery and display the prompt "BitLocker needs your recovery key to unlock your drive because Secure Boot policy has unexpectedly changed" after installation of KB5012170 (Security Update for Secure Boot DBX: August 9, 2022).
Unfortunately, there are certain instances where BitLocker drive encryption can be enabled without the user's knowledge [e.g., during the initial "out-of-box experience" (OOBE) setup on Win 10 Pro / Win 11 Pro or on some devices that support Modern Standby sleep mode - see my 16-Aug-2022 post in ecarpenter's Inspiron 7391 BIOS Update Enabled Bitlocker in the Dell forum], and these Win 11 users will not have easy access to their BitLocker recovery key after applying KB5012170.
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From Microsoft's Windows 11 Known Issues and Notifications for August 2022 (revised 19-Aug-2022):
Some devices might enter BitLocker Recovery on the first or second restart after attempting to install Security update for Secure Boot DBX (KB5012170), released August 9, 2022. Note: This issue only affects the Security update for Secure Boot DBX (KB5012170) and does not affect the latest cumulative security updates, monthly rollups, or security only updates released on August 9, 2022.
Workaround: If your device is prompting for a BitLocker Recovery key, you will need to supply it to start up Windows. For more information, see Finding your BitLocker recovery key in Windows.
If you have not installed KB5012170 yet and have BitLocker enabled on your device, follow the instructions below to temporarily suspend BitLocker before installing.
If you have installed KB5012170 and have not yet restarted your device or have only restarted your device once, temporarily suspend BitLocker using the instructions below.
Important: If you have restarted your device two times or more after installing KB5012170, your device is not affected by this issue
To temporarily suspend BitLocker, or to avoid a BitLocker recovery when deploying KB5012170, follow these steps: ...
Kudos to Alex5723 for posting a link to that Microsoft Docs support article in post # 2471483 of Susan Bradley's topic It’s Time For Those August Updates To Be Deferred in the AskWoody.com forum.
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Dell Inspiron 5584 * 64-bit Win 10 Pro v21H2 build 19044.1889 * Firefox v103.0.2 * Microsoft Defender v4.18.2205.7-1.1.19500.2 * Malwarebytes Premium v4.5.13.208-1.0.1740 * Macrium Reflect Free v8.0.6867