Keep in mind, COVID-19 causes are a small fraction of Myocarditis.
In 2013, about 1.5 million cases of acute myocarditis occurred. While people of all ages are affected, the young are most often affected
(wiki source)
https://en.wikipedia.org/wiki/Myocarditis
Infections
Viral: adenovirus, parvovirus B19, coxsackie virus, rubella virus, polio virus, Epstein-Barr virus, hepatitis C, severe acute respiratory syndrome coronavirus 2
Protozoan: Trypanosoma cruzi (causing Chagas disease) and Toxoplasma gondii
Bacterial: Brucella, Corynebacterium diphtheriae, gonococcus, Haemophilus influenzae, Actinomyces, Tropheryma whipplei, Vibrio cholerae, Borrelia burgdorferi, leptospirosis, Rickettsia, Mycoplasma pneumoniae
Fungal: Aspergillus
Parasitic: ascaris, Echinococcus granulosus, Paragonimus westermani, schistosoma, Taenia solium, Trichinella spiralis, visceral larva migrans, Wuchereria bancrofti
Bacterial myocarditis is rare in patients without immunodeficiency.
Toxins
Drugs, including alcohol, anthracyclines and some other forms of chemotherapy, and antipsychotics, e.g., clozapine, also some designer drugs such as mephedrone
Immunologic
Allergic (acetazolamide, amitriptyline)
Rejection after a heart transplant
Autoantigens (scleroderma, systemic lupus erythematosus, sarcoidosis, systemic vasculitis such as eosinophilic granulomatosis with polyangiitis, and granulomatosis with polyangiitis, Kawasaki disease, idiopathic hypereosinophilic syndrome)
Toxins (arsenic, toxic shock syndrome toxin, carbon monoxide, or snake venom)
Heavy metals (copper or iron)
Physical agents
Electric shock, hyperpyrexia, and radiation
My wife had died due to a myocarditis. Unknown causes, long before COVID pandemic.