I have a MedTronics device for almost thirteen years, not for my heart but implanted on my brain. The only situation I was warned for, are security gates at airports and medical devices such as MRI scanners.
After it was placed, I received a strong magnet to shutdown the device in case it should go haywire. The magnet should be placed on the pacemaker and it should down by itself (never tested though).
Now I'm eligable for a new phone and I was planning to get an iPhone 12 Pro. I've read the stories about the magnets too. And I remember stories of the iPad cases triggering medical devices too. I have had iPads since its first release with Apple cases and I never had issues with my pacemaker. However, I have no experience with Apple's Smart Cover which has magnets to keep the cover on the iPad.
Now, to make a long story short, I decided not to get an iPhone 12 because of the magnets but to stay on the Android route (sigh).