For the past couple of weeks, Windows 10 users have been reporting that Windows Defender scans are skipping files due to a configured exclusion or network scanning setting. A notification about skipped files appears for users who have no exclusion configured.
However, there are no exclusions configured in Windows Security, as shown in the screenshot below.
The issue is not confirmed by Microsoft yet, so there is no solution available at this moment.
According to research by Bleeping Computer, Defender has started misbehaving on March 10th, 2020, after the corresponding Patch Tuesday.
The issue affects the anti-malware client version 4.18.2003.6, and above.