This is not to discount the effects or concerns about it -- just curiosity.
But we do seem to see an uptick not only amongst the infected but also amongst the vaccinated (note: I do believe that infection is much worse, but vaccinated to infected are roughly 80:20, so it tends to look kind of even). What if the risk of myocarditis was not so much caused by coincidence of training intensity and immune stress but also by coincidence of immune stress and whatever happens during training recovery (when the body does the actual work of adaption, you don't get better during training, you get better afterwards)? When you tell an athlete that they get their shot on Wednesday and they should better take some rest days afterwards, it's as predictable as Wham in December that they will throw in some serious training load on Tuesday. Chances are they'll train extra hard so that they need those rest days anyways. (for me it was Sunday/Monday)
Now the thing I find particularly interesting about this speculation: if this mechanism was real, myocarditis risk not from high training load coinciding with immune stress but/also from immune stress coinciding with post-load recovery, it would be highly unlikely that we'd know about it. Because with naturally occurring immune stress, we don't ever have a clearly defined point in time when it started, and we certainly wouldn't ever deliberately schedule it so that we could arrange for training load to happen right before.
This study shows >2x risk for previously infected healthcare workers.
About 105 vaccine recipients reported at least one AEFI symptoms following COVID-19 vaccination (incidence proportion 6.4%, 95% CI: 5.3%, 7.7%). All AEFI reported were minor which were managed by tablet paracetamol and subsided after 1–2 days. Fever (65, 3.98%) was the most commonly reported AEFI followed by myalgia (54, 3.30%) (Fig. 1). About 48 (2.94%) study participants reported 2 or more AEFI, most common being fever with myalgia (19, 1.16%) and fever with headache (18, 1.10%). No severe or serious AEFI was reported among vaccine recipients.