Drupal 8 REST export View shows “This display has no path” in View UI and 404 Error on page

I’ve got a fresh install of Drupal 8.6.x with REST/API/JSON Web Services enabled and I’ve created a Rest export View with path: /api/test which returns the correct JSON View results when I append the required querystring:

?_format=json 

So all is well. Until I either create a second REST export View or a new Display in the working View. These seem to work fine in the View’s UI Preview but also displays “This display has no path” just above the Preview output. If I go to the new View/Display’s URL I get Page Not Found and if I append ?_format=json I get this:

{ "message": "No route found for \"GET /api/temp-view\"" } 

(/api/temp-view is my second View’s path).

It’s the strangest thing. Only the first, original view Display is working. I’ve created all sorts of different URL paths to test (and Saved each time). The View settings are standard and identical.

I’ve spent about 6 hours on this now…

Any help much appreciated.