That is part of the condition to get full FDA approval but they will probably not follow through, just as they haven't done any of the biennial FDA-required safety testing on childhood vaccines since it was mandated back in the 80's.
We really don't need them to do a myocarditis study anyway since there is a very high quality prospective RCT from Thailand that shows about a 2.3% incidence of myocarditis/pericarditis in healthy young people after receiving the
safe and effective™ mRNA jabs. This represents a massive several hundred fold increased risk of myocarditis/pericarditis compared to a non-jabbed population:
Asking Pfizer and Moderna to do myocarditis risk studies on their "vaccines" would be like asking Philip Morris to do a study on the risk of lung cancer from their cigarettes, totally pointless and not likely to give you an unbiased result.