Hmm, I've not heard of a different manufacturing process between vaccines administered for the trial participants and general public. If you have a source on that, it'd be helpful.

As the other commenter said, there're other ways to determine whether vaccines cause a particular condition like myocarditis, such as the consistency of statistically significant and strong (i.e., high effect size) association across multiple countries. I believe that was how scientists determined smoking causes lung cancer. I also think randomized trials are less helpful in this case because it's quite unfeasible to recruit a sample size large enough and long enough to detect PVS.