Earlier in January, a medical study done by the Heart Rhythm Society (first spotted by MacMagazine) suggested the iPhone 12 and MagSafe accessories can interfere with medical devices - such as defibrillators and pacemakers. This isn't new information as most smartphones and gadgets come with magnets, and these tend to interfere with pacemakers and other medical implants if kept too close.
The document also mentions both the MagSafe Charger and the MagSafe Duo specifically, confirming that while all MagSafe accessories contain magnets, the "MagSafe Charger and MagSafe Duo Charger contain radios". However it is advisable to keep the devices including Apple's Magsafe chargers at a safe distance from medical devices. To avoid possible contact with these devices, keep your iPhone and Maxoff jewelry at a safe distance from the device (more than 6 inches / 15 cm or more than 12 inches / 30 cm if you charge wirelessly). "Though all iPhone 12 models contain more magnets than prior iPhone models, they're not expected to pose a greater risk of magnetic interference to medical devices than prior iPhone models", Apple says in the support document (via Macrumors). The latter states that manufacturers like Medtronics could reduce the use of magnetics or use some sort of method that would prevent interference from ambient sources to impact life sustaining medical devices like a pacemaker. This comes after a research titled Life Saving Therapy Inhibition by Phones Containing Magnets was published and said, "We hereby bring an important public health issue concerning the newer generation iPhone 12 which can potentially inhibit lifesaving therapy in a patient particularly while carrying the phone in upper pockets".
Further it commends that users can consult with their physician and device manufacturers for specific guidelines since these can differ. It is therefore recommended that one should consult a medical expert in detail regarding all the do's and dont's to ensure that their daily use devices such as phones, smartwatches, or wireless chargers do not interfere with the functioning of their life-saving medical devices. Apple repeated this advice in the safety information section of its iPhone User Guide.
In a way, we can conclude that the iPhone 12 and other types with MacSafe do not cause additional problems if used with some caution by a person with a heart attack.