List View Screwed up (calendar)

Hello,

Yesterday a single user reported that their calendar view on a list was screwed up. The following image is how the view should look, and how it does look for our hundreds of users. 

This single user though, now sees exactly the same view, but displayed like this:

I have highlighted the area of concern in red. Views are no longer in the dropdown, but are shown in a tab form. When the user views the list in Microsoft Lists, the views are still shown in a tab view, as they are here. 

On top of that, the filters of the current view are no longer working for the user, in fact, as of today, the user does not see ANY data in this list. 

Things I have tried / checked:
 - I have checked the permissions, and the user is in the same group as everyone else. An 'all users group'.
- I did a remote session with the user and: signed them out and in, cleared their browser cache, and signed in as myself. When I signed in, the list was displayed correctly. 

- I have done some searching, and found mentions of lists now opening in Lists by default. There was a suggestion to enable the 'Show header and navigation' setting in Navigation Elements in Site Settings, which I have also done, with no effect.

Please, does anyone have any idea what is happening here? I have nothing to offer except my eternal gratitude. Thank you in advance.