That’s a standardized romanization though; in traditional Hepburn, if n is before b, m, or p, then it’s romanized as m instead. Other romanizations keep it as n, but that doesn’t mean “sempai” is wrong
This.
I was meaning address the original post about this, but I got a bit busy. However it's not just the fault of Hepburn, when "ん" is before b, m or p, it is genuinely pronounced as "m", and it is merely convention that it is transliterated as "n". A basic word such as "zenbu" (all, lit. all of a part) is actually pronounced "zembu" for example, but, long ago, when I was in Jap 101 it was romanised as "zenbu" (before we were secure enough with hiragana to dispense with romanisation).
There is an argument to be made from sticking to convention for the purposes of accessibility though. Recently, I tend to favour spellings such as "favor" "mom" "color", because despite those spellings being incorrect in my eyes, I understand that the majority of the audience is comfortable with that spelling. Why shouldn't that argument extend to senpai vs sempai? I don't have an answer to that, but I have no intention of stopping lol
I used to be on the other side of this argument btw.
Thanks for the explanation btw, RiversofItaly, I just wanted to chuck my 2 cents in.