The ACA is not an insurance plan, so I assume you are using "ACA" as shorthand for "ACA-compliant insurance coverage".
It is simply not plausible that an ACA-compliant plan would wholesale "not cover" a hospital stay for myocarditis, absent extremely niche and bizarre circumstances.
Given that you don't seem very familiar with the specifics, I have to assume that your information is either missing some very important details, or incorrect.
As a lifelong heart patient, I've had many post-operative stays not covered, past a certain duration regardless of my current condition and I've always paid for the highest tiers of health coverage (for over 2 decades now). I find it very likely that ACA would not cover" a hospital stay for myocarditis.
Suppose the insured did use a non evidence supported vaccine or whatever, the vaccine itself would not be covered by insurance, but the healthcare needed by the damage caused by the use of unsupported medicine would be covered.
Similarly, health insurance has to pay for your healthcare if you get into a car accident and do not have sufficient auto insurance to pay for your healthcare costs. Technically, you are sort of buying redundant insurance if you buy PIP.
Falling while skiing is not FDA approved. A woodcutting axe applied to the hand is not FDA approved. Drinking bleach is not FDA approved. And yet, health insurance will cover medical complications arising from any of these actions.