Classic SharePoint features appearing on a Modern Site

I have a modern SharePoint site, with a modern list that has never been associated with classic SharePoint features.

But today, two users have experienced intermittent instances of the list appearing in the classic interface. They have not accessed this site before now so I do not believe there should be a cache issue.

Does anyone know why this is happening and how it's possible when this site has no association with Classic features?  They are using the same browser, same permissions across the site but sometimes the list opens in Classic mode, sometimes it opens in Modern. It also is happening in incognito mode. They are unable to use the site while in Classic Interface at all - the people pickers deny them access ("Error: Sorry, you do not have permission to query for users" and they cannot edit anything because of this error.