You do not need to install a third-party antivirus.
Check the "Resolved Threats" and "Protection Log". If everything is in order there, as well as in other sections of "No action required", then this is just a bug in the Defender interface (as I said earlier, this happens) due to the launch time of the security services. Well, just in case, perform a scan with Offline Defender (click on "Scan Options", select "offline scan" there and click on "Scan now"). The PC will reboot and the Defender will perform a check. If there are any threats, notifications will appear on the screen. If there are no notifications, it means that everything is in order and there is no cause for concern.