Lens-position sensors respond to magnetic fields. If you place a magnet near these sensors, the magnetic field will interfere with or temporarily disable the sensors. This can degrade the sensors’ accuracy and limit the range of movement available to the lenses. The camera will continue to take photos with other means of stabilization but without the benefit of OIS and closed-loop AF.
Third-party manufacturers make some accessories with strong magnets or magnetizable metal plates that are near the iPhone rear camera (or cameras). These magnets and plates can latch folio covers, join separable cases, or attach to rigid mounts—for example, car mounts. For best camera performance, avoid accessories that use magnets or magnetic metal near the iPhone rear camera (or cameras).