THE CRUNCH
Microsoft has patched a bug in Windows Defender that falsely reported the antivirus was turned off. The issue, which affected supported Windows versions, was resolved by Defender platform update 4.18.26080.4 released on September 17. The alert appeared at random, even with notifications disabled, and did not affect the actual protection status.
Microsoft acknowledged the problem on August 28, but the fix took nearly three weeks to arrive. The update is automatic unless you have blocked mandatory security updates through Group Policy or the Registry Editor.
Windows Insider users reported similar warnings since at least June. The bug was particularly annoying because clicking the alert only opened the Windows Security home tab, where everything showed as active.