It is a case of 'ㄴ 첨가 현상'. When a word (esp. a noun) starting with a /j/ sound (이, 야, 얘, 여, 예, 요, 유) comes after a syllable ending in a consonant, ㄴ is added to the word to make it 니, 냐, ... etc. This happens both across different words and between different parts of compound words. 찻잎 is a compound word of 차(tea) and 잎(leaf) with a 사이시옷 (in-between ㅅ) that smooths the formation, so it triggers this ㄴ 첨가 phenomenon. ㄴ 첨가 occurs because we stress the beginning of new concepts when we speak. When the 이(/j/) sound receives a stress after a consonant, ㄴ is subconsciously added to indicate a start of a new concept. To be precise, 찻잎 is first conceptualized as /찯 + 입/ (because both 차 and 잎 are common words/concepts), then it becomes /찯닙/ to make 잎 distinct, and subsequently /찬닢/ because ㄷ-ㄴ always degenerates to ㄴ-ㄴ for smoother pronunciation. ㄴ 첨가 is one of the most complex (and misused) phonetic rules in Korean, because word or concept boundaries are not always clear even to Koreans. There are numerous words that are pronounced differently by different people because of this. e.g. (ㄴ 첨가 cases) - 일본 요리 (일본 + 요리) -> /일본뇨리/. (noun phrase) - 이상한 일 -> /이상한 닐/. (adj + noun case) - 나뭇잎 (나무 + ㅅ + 잎) -> /나문닙/. (compound noun) - 먼일 (먼 + 일) -> /먼닐/. (compound noun) (no ㄴ 첨가 cases) - 만일에 -> /마니레/ (만일 is considered a unit word) - 젊은이 -> /절므니/ (이 is a suffix here, not an independent word) - 절반이다 -> /절바니다/ (이다 and other particles are like suffixes, not independent words) - 안일 -> /아닐/, 은유 -> /으뉴/ (hanja words are not compound words) (frequently mispronounced cases) - 손예진 (a name) -> /소녜진/ or /손녜진/ (the former is correct, but both pronunciations are common) - 윤석열 (a name) -> /윤서결/ or /윤성녈/ (same)

@KaySong: Thank you so much!

I think that is 사잇소리현상. Please check this url below. 꽃-잎[꼰닙] 색-연필[생년필] https://ko.wikipedia.org/wiki/%EC%82%AC%EC%9E%87%EC%86%8C%EB%A6%AC_%ED%98%84%EC%83%81#%EC%82%AC%EC%9E%87%EC%86%8C%EB%A6%AC%EC%9D%98_%EC%A2%85%EB%A5%98_2