https://www.bleepingcomputer.com/news/microsoft/windows-10-1909-kb5003212-cumulative-update-preview-released/

Microsoft has released the optional KB5003212 preview cumulative update for Windows 10 1909 with bug fixes and performance optimizations in the operating system.

This cumulative update is Microsoft's May 2021 monthly "C" preview update that allows users and admins to test out the upcoming fixes scheduled for release in the forthcoming June 2021 Patch Tuesday.