When using relative positioning of controls, the X and or Y properties are set to a formula. It is very easy to attempt to select a control that is using relative positioning by clicking on it, and accidentally move it, which replaces the relative formula with an absolute value.
I propose that there is a "Lock Relative Controls" flag in the Canvas Apps Studio that prevents any control with a formula for an X and/or Y from being manually repositioned unless the the "Lock Relative Controls" flag is disabled. Ideally this flag would exist on the App or screen level, applying to all controls on the app/screen that are using relative positioning. Bonus points if you can still reposition the control the control along any axis that is not using relative positioning.