Ok so the actual word in yojinbo, and whatever Heburn translation was used makes it an m instead.
More helpful was the tiny little question mark by the romaji word which states the rule:
An n before a consonant is moraic (its own mora).
A moraic n followed by a vowel or y is written n'.
The moraic n has various phonetic realisations:
Before an n, t, d or r, it is pronounced [n].
Before a k or g, it is pronounced [ŋ].
Before an m, b or p, it is pronounced as [m]. It is written as m in some versions of Hepburn, but as n in Wikipedia's modified Hepburn.
Matthew Zsebik wrote:
Now you added 'u' to 'yo' and 'bo' to denote the long 'o' sound, making the word 'youjinbou'.
I thought adding the u (ou or oo in old Hepburn) was just a romaji notation to help english speakers, not actually part of the Japanese word!?!