I've never seen this before. Our global and site level settings are 'don't allow sharing outside your organization' but today I noticed in the Site Permissions page an entry for Anonymous Users. It is not clickable and each time I check the box, it checks all other boxes. Does it impact our global settings? Or, how do I remove this permission?
Did the issue occur in SharePoint online?
If so, I suggest you could remove the “Anonymous Users” from server end. You could submit a service request like the picture below.
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