Who's the football player with heart failure after getting vaccinated? We have multiple instances of players getting Covid and having complications from that ( Kimmich, Aubemayang, Davies, the first one of which is certainly unvaccinated, no idea about the rest), but i haven't heard of heart failure after vaccination. If you mean Christian Eriksen, he wasn't vaccinated at that time.
Incredibly fit young people sometimes die of heart attacks. They had no diagnosible heart condition prior. There are a couple hundred ways the heart can fail, and only the common ones are tested for.
I.e. being fit and healthy will reduce the odds of a heart attack, but will never eliminate the possibility.