Android 12: Disable Floating Niagara Button
If you upgraded to Android 12 and enabled the accessibility service for Niagara Launcher for the double-tap-to-lock-screen feature, you may have noticed this floating button/bubble appearing on the screen. This is the new accessibility shortcut button of Android 12 but it isn't useful for locking the screen.
To turn it off, open Android's settings > Accessibility > Niagara Launcher > turn off Niagara Launcher shortcut.