I'm pretty sure it's common knowledge that the title Unknown (Till The End...) has a lot of meanings behind it, but I'm just listing them here because I find it really cool how you can take away a lot of things just from these few words.
1) The first and most obvious one is that it's Till's original song, one that nobody expected him to perform. It's most likely not recognized unlike the other songs (which seem to be pre-made or at least partly produced by aliens), therefore registering in the system as Unknown while Till The End... is most likely the title Till actually intended for it.
2) Secondly, it could be referencing Till's unrequited feelings for Mizi, who he wrote the song for. His love is unknown until the end, further emphasized by his lyric:
The edelweiss of my feelings that blossomed because of you (you'll never know...)
It could also state that Till believes his feelings for Mizi to be everlasting. Without end, just like how an edelweiss flower symbolizes perseverance and devotion.
3) The third meaning (which VIVINOS likes most), is that one of the Korean translations for unknown is 미지 (Miji), which is how Mizi's name is written in Korean. Till essentially named the song "Mizi (Till The End...)".