How do people know? For every previous vaccine, myocarditis inflammation has mostly been temporary. If this one is different, how would people know it's lifelong?

It's much, much more likely that reaction to the vaccine is uncovering existing, latent myocarditis than it is causing them. A la https://en.wikipedia.org/wiki/Seattle_windshield_pitting_epi...

Also, nobody who cares about myocarditis seems to be pointing out that you are 16x more likely to get a myocarditis diagnosis from catching Covid than you are for getting vaccinated.