Hi! So far what i know korean names when called informally they add some vowels to sound it natural or maybe any other reason like. Minjia, seoyeona,sungyeola, taehyungah,jimina etc
If the last syllable is consonant then we use 아 and if the last syllable ends with a vowel then we use 야
For e.g. 1.Jimin = jimina
2. Hoseok = hoseoka
1.Mina = Minaya
2.Yoongi = yoongiya
I hope this helped