Odd. Using a 21:9 monitor on my main dev machine myself. Changing the aspect ratio of the actual display should not affect the virtual monitor. Can’t say anything useful currently, but I’ll try to replicate the issue. Thanks for the heads-up. If you don’t want to use the virtual monitor, you can disable it entirely in the config app.
Unrelated sidenote: should you incidentally encounter a supported game not launching with a required DirectX version, it might be worth a shot not to ignore warnings related to disabling automatic settings. As these warnings suggest, disabling automatic settings can (and often does) break important stuff.