Windows Defender alerts state that items were skipped when they performed scans using Windows Defender. G..nter Born who first reported about this issue has now discovered a fix that just entails enabling the scanning of network files. Microsoft states in their documentation that scanning network files is not recommended, but leaving scanning disabled will continue to display these alerts. It is not known why Microsoft decided to make this change, but the alerts appear to just indicate that network scanning was skipped. In an older Windows 10 VM from right before the March Patch Tuesday updates, using the Get-MpPreference clearly shows network scanning is enabled.
Source: https://www.bleepingcomputer.com/news/security/windows-defender-fix-for-windows-10-enable-network-scanning/