Scaling

This is an issue with either your Anti-Aliasing or Resolution which causes your game to render chams at an offset.

Resolution

To fix this, open Rainbow Six Siege settings, go to the Display tab, and set the resolution to your native display resolution.

Anti-Aliasing

To fix this, open Rainbow Six Siege settings, go to the Graphics tab, and scroll down until you find Anti-Aliasing. Set it to FXAA or Off.