You have a good point. First, I think it's possible for the mRNA vaccine to result in higher circulating spike (full-length) and not S1. This would mean that the circulating spike was not due to S1-S2 cleavage but another mechanism, such as cytotoxic T-cell responses. But because antibody and T-cell responses were similar between vaccinees with and without myocarditis and circulating spike, I figured different rates of spike protein production is likely the culprit.

But I agree that the different time of sample collection is a flaw of the study. Even so, figure 4 of the study does not seem to suggest any significant time-dependent changes in circulating full-length spike.

Regarding Novavax, you are right in that spike protein overproduction does not explain why some people who got this vaccine developed myocarditis. I did not think of this. I will add an addendum to clarify this.

And my apologies for the late response as I was unwell for the past few days.