https://www.bleepingcomputer.com/news/microsoft/windows-10-secure-boot-update-triggers-bitlocker-key-recovery/

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.

Secure Boot is a security feature that blocks untrusted operating systems bootloaders on computers with Unified Extensible Firmware Interface (UEFI) firmware and a Trusted Platform Module (TPM) chip to prevent rootkits from loading during the OS startup process.