'전' translates as before or ago
내가 집에 오기 전에 = Before I came home
5 년 전에 - 5 years ago
'까지' translates as to or until/ a place or time
100까지 센다 = count to 100
When 전 is placed before 까지 as in 전까지 it means before this specific time.
5 시 전까지 만나자 = Let's meet before 5 o'clock
전까진 is not interchangeable with 전에 and has a different nuance.
전까진 means 'all' the time before or leading up because it is connected with 까지 which means 'as far as' or 'till'.
While 전 just means 'before' or 'ago.' So the nuance of the two sentences are different in how they convey 'before'
내 문제가 되기 전까진 본 적도 없어요
Has the nuance of 'In all the time before and up until' or even 'In all my life' before it became my problem, I never experienced it. While
내 문제가 되기 전에 본 적도 없어요
Has the nuance of 'The time before' it became my problem I never experienced it.