I've got a few apps, and I have been able to set a max width of 1200px in the app settings for all of my apps except one. On one of my apps, I set the max width to 1200px, and it shows that the settings saves, but my canvas is still 100% width. The same thing is happening in both preview mode, and when actually viewing the app.
Hm! I can't reproduce this on our end. Do you have any custom CSS? If you'd like to PM your Retool domain name, I can also see if there have been any recent changes to your organization that might have caused this.
I haven't added any custom CSS. Sure, I can send my domain name. I'm having trouble finding where to PM you, though.
Followed up via a DM. You should get a notification.