/n/ to /m/ transition before a bilabial consonant (in this case, /p/) is fairly common phenomenon in a variety of languages. Note that this happens in Japanese

/n/ to /m/ transition before a bilabial consonant (in this case, /p/) is fairly common phenomenon in a variety of languages. Note that this happens in Japanese when /n/ (ん) occurs before syllables starting with /b/ (shimbun 新聞), /p/ (sampo 散歩), and /m/ (sammai 三枚). Also note that there are regional, socioeconomic, and register differences regarding whether/when this /n/ to /m/ transition occurs in a speaker's actual pronunciation.