My surname beings with Mac- but there isn't a capital letter after the Mac. For the purpose of illustration I'm going to use "Macsomething". Not MacSomething and certainly not McSomething. The lack of a capital letter is not some faddy thing we've made up, it's been spelt that way in DH's family for at least 4 generations that we know of. DH was using the initial M for his surname when I first knew him (though he doesn't very often need a surname initial as his first name is fairly rare in our age group).
I work in a place where there are other people with the same first name as me. I've been there for about 10 years and have always signed myself "Username M" when I need to use an initial but invariably when someone else writes my name, they write "Username Mac" or even more annoyingly, "Username Mc" or very occasionally "Username Mac/McS". It's not necessary as there isn't another Username M. I haven't made a big deal of it, just continued to use M myself, but it slightly pisses me off every time someone adds the unnecessary letters. They don't call Username Harrison (name changed!) "Username Har", or Username Przewalska "Username Prz"!
As I said, I haven't actually complained about this because I half think it would just be petty, especially after so long, but AIBU just to find it a bit annoying?