Eh while it sucks, registrars and web hosts get so many abuse reports that sometimes they just err on the side of caution and don’t investigate as thoroughly as you’d like.
Of course it also depends a lot on various things like what type of complaint, how much money you spend with them, account history, complaint source, etc.
They should be able to tell you what they had a problem with and give you a chance to fix it.
Why mtls support specifically? You could use any web based notes app (with PWA) and have the web server / reverse proxy handle the mtls part.