I'll give some context here, since I put magnets on people's pacemakers fairly often. Magnets for at least 10 years now is a standard, controlled way to trigger a secondary mode in a pacemaker/defibrillator. Almost always, if the device has both pacemaking and defibrillating capability, the magnet will turn off the ICD (although it is possible to program other behaviors). This allows clinicians to easily stop a patient from being defibrillated inappropriately. That sort of thing could happen during malfunction, electrical interference from cautery tools during surgery near the device, or refractory shockable rhythms.
If there is no defibrillating capability, magnets usually (although not always) put the device into an asynchronous mode. This presents the (extremely unlikely) risk of something serious, like R-on-T, where an electric pulse could be sent to the heart at exactly the wrong time, leading to a life-threatening arrhythmia. More commonly, it means the heart is less effective at pumping blood but probably still within allowable, if not optimal for long periods of time, output.
In both cases, unless the device is programmed strangely, the device resumes normal function when removing the magnet. However, there's the theoretical risk of some sort of malfunction-- so when switching modes with a magnet my practice is to refer patients to getting their devices checked out afterwards. Obviously, if something triggered the "magnet mode" surreptitiously, it may not be noticed until some time later, particularly in patients who are not being actively followed by medical care.
I didn't mention it's actually kind of difficult to getting a magent over these things. I generally tape it over the area that the pacemaker is in, but you have to watch for it moving because, well, it's hard to stick something on somebody's skin. Pacemakers are generally put in the left upper chest, sort of between the nipple and the clavicle. The idea you should have is-- well-- it's not easy to trigger this even with a purpose-built magnet in a slightly-higher-then normal breast pocket. But, still possible of course, and the results could be disastrous, particularly in the event of a malfunction.
The practice of using magnets is kind of universally and internationally understood by medical practitioners and device manufacturers. Which means, transitioning away could cause some harm just by medical errors. I don't think a solution to this is particularly easy, although many devices now get regular interrogations which would show if iPhones actually are triggering the mode switch. Which sort of means, iPhone "interference" could happen but probably isn't really happening as far as anyone knows.