Windows 11 Preview Update Breaks Mouse Settings and Triggers False Defender Alerts

Mouse Cursor Anomalies and Desktop Instability

The most immediate impact of KB5120998 is observed in the behavior of the mouse pointer. Users reporting the issue describe a regression where custom cursor shapes and animations revert to default settings unexpectedly. In some cases, high-resolution cursors have been reported to change appearance abruptly, becoming larger or switching to a stark white color scheme.

Beyond the cursor itself, the update has triggered broader desktop instability. A subset of users on Reddit and other forums noted that their desktop backgrounds vanished entirely, reverting to a solid black screen. Microsoft has explicitly stated that while they are aware of these reports, they have not yet confirmed whether this specific preview build is the direct cause of background disappearance or if it stems from a separate compatibility issue.

Microsoft Defender False Positives

A distinct but equally disruptive bug involves Microsoft Defender Antivirus. Unlike the mouse issues, which appear tied to the Windows client update itself, this problem manifests as false positive alerts unrelated to system security status. Following the installation of the update, users began receiving notifications stating that "Microsoft Defender Antivirus is turned off."

This is misleading, as the antivirus software remains active and functional despite the warning. Microsoft has highlighted that this specific annoyance occurs repeatedly upon every system restart for some users. The company notes that while the issue may seem minor, the constant notification loop creates a significant usability burden, particularly when the user must manually dismiss alerts only to have them reappear after rebooting.

Scope and Investigation Status

Microsoft has clarified the scope of these regressions. The mouse cursor and desktop background issues are confirmed to affect Windows 11 client versions (24H2 and 25H2) but do not impact server editions of the operating system. Regarding the Defender alerts, Microsoft indicates that this bug affects all supported client and server versions of both Windows 10 and Windows 11.

The company has requested user feedback via the Feedback Hub to gather additional data necessary for debugging these regressions. However, Microsoft has not definitively admitted fault, stating only that they are currently investigating the root causes. Until a fix is deployed or a specific workaround is confirmed, users on the Windows 11 preview channel should expect potential instability with input devices and persistent security notifications.