Some seems to be having problems with this Windows Security registration. ESET had this issue for some users for a few months. Kaspersky has an issue now but that's a bit different. If you uninstall Kaspersky, it doesn't remove itself from Windows Security registration. The main uninstaller and their uninstallation tool (which is worse than the default uninstaller) both don't remove it, so after uninstallation even Windows Defender doesn't start as the AV. Good for me, I know how to manually fix that. Some AV vendors really need to fix their Windows Security integration. Maybe Windows change things from time to time and this cause problems for them.
Edit: I'm on 21H1. So don't know if that has something to do with this.