Microsoft has acknowledged an issue affecting Windows 10 customers who have installed the KB4535680 security update that addresses a security feature bypass vulnerability in Secure Boot. Windows versions affected by this vulnerability include multiple Windows 10 releases (from v1607 to v1909) Windows 8.1, Windows Server 2012 R2, and Windows Server Server 2012. Microsoft recommends temporarily pausing BitLocker before installing the update acts as a workaround for the issue. Users experiencing the issue can use the information provided by Microsoft here to locate their recovery key.
Source: https://www.bleepingcomputer.com/news/microsoft/windows-10-secure-boot-update-triggers-bitlocker-key-recovery/