You guys.
The apostrophe has nothing to do with whether the ん sounds "nasally" or not.
It's purely a function of transliterating hiragana into roumaji. The idea is that you should, in theory, be able to transliterate it bidirectionally, i.e. back from roumaji into hiragana as well.
Take a roumaji word such as "kinen." How do you write that in hiragana? The answer is ambiguous because it could be either the four kana きんえん (no smoking) or the three kana きねん (anniversary). The sole purpose of the apostrophe is to remove that ambiguity. Kin'en definitely has four kana, not three, and hence means 禁煙 (no smoking). Similarly with ren'ai. It is clearly れんあい and can't be mistaken for れない. The fact that Japanese people in some contexts give ん a uvular or vocalic quality is an entirely separate issue from the placement of the apostrophe.
Anyway, the apostrophe is only a rule in some romanization systems (notably Revised Hepburn) which is why you see it used some times and not others. Expecting consistency all of a sudden would be like saying it can only be Berryz Kobo and not Koubou or Kōbō.