My apologies for my late response. While I have not done in-depth reading into the mechanisms of post-mRNA vaccine heart problems, I think this review paper (section 3) discussed a few plausible hypotheses, such as excessive inflammation generated by the immune system of young people.

https://www.mdpi.com/2227-9067/8/7/607

As far as I know, mRNA vaccines are not known to cause thrombosis (blood clots). Only the adenoviral-vectored DNA-based vaccines have been acknowledged to cause VITT (vaccine-induced thrombotic thrombocytopenia).