There are two distinct pronunciations, げんいん and げいいん, the first is the standard, the second is a variant. The only consequence of the ん in the second one is that it has been replaced because it's a pain to pronounce in that position. That there are other words with ん in a similar position (一千円 not being one of them) is by the by, 原因 and 雰囲気 are two of most frequently used words in the language (in the first 800), 禁煙 is used far less (in the first 5000) so under less pressure for adaptation.
