https://www.bleepingcomputer.com/news/microsoft/windows-10-1909-kb5001396-cumulative-update-preview-released/

Microsoft has released the optional KB5001396 preview cumulative update for Windows 10 1909 with numerous fixes that increase performance in the operating system.

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