UI not scaling correctly with screen resolution

When I run my game in the editor and change the resolution using the editor dropdown, everything works fine

(1280x800)

(720x450)

The window also resizes correctly when I build my game and exit fullscreen.
However, when I switch to fullscreen from ingame, the UI does not scale with the screen size unless I set the resolution to the native resolution. (not allowed any more pictures, sorry about link.)

Album

Canvas settings

Part of fullscreen/resolution change script:

 void ChangeSettings () {
         //...
         StartCoroutine (Fullscreen (isFullscreen)); //isFullscreen = Player Pref
         //...
     }
 
     private IEnumerator Fullscreen (bool fullscreen) {
         int width = Mathf.FloorToInt (resolution.x); //Player pref resolution
         int height = Mathf.FloorToInt (resolution.y);
         Screen.fullScreen = fullscreen;
 
         yield return new WaitForEndOfFrame ();
         yield return new WaitForEndOfFrame ();
 
         Screen.SetResolution (width, height, Screen.fullScreen);
     }