It appears every few times I open the app and the usual android back button doesn’t make it go away, and there is no cancel button. What you have to do is toggle the toggle, then the button will change to “do not back up” and then you can press it. It annoys me how easy this makes it to accidentally turn on backup.
It’s ridiculous, its enabled by default on any new device you set up and sign in to. I could have missed turning off iCloud on my iMac, but I didn’t have any problems before updating it, and then after I updated it I had iCloud full notifications, so I’m pretty sure it was off.