Yes, thankfully. I had to contact the Evernote team on Twitter and it led to this answer.
I was using the Teams app on a Macbook. My preferred browser is Firefox, but I wasn't using Evernote on Firefox. However, this is what they sent me:
"Additionally, when you log into Evernote on the web (please test this on Safari, if on Mac, and Internet Explorer, if on Windows), do you have an all-white interface, or do you have a black left-hand panel? The black left-hand panel is the latest web version and it will be necessary to use MS Teams properly. Since Safari and IE are the browsers Evernote uses to connect to the service, this is why I ask."
When I opened Safari and logged in, the interface was all-white. I then updated it on Safari and the new black sidebar appeared. As a result, it all started working in Teams app on the Macbook, so all sorted now.