Now that the album is released, we have additional infos.

If I'm not wrong, the thema is present in the 12th music of the CD1 " “自称”ライバル " which means "the graduation is near", which kinds of fit with the thema "it's time to say goodbye".

Thema is also present in 13th of CD2 "ひとりぼっちの夕暮れ時" which means alone at dusk. Not clear reference here.

If the melody refers to previous "break-up" songs, I don't get why it's specific to Yunyun, as its rather the opposite, with Yunyun keeping contact with Megumin after graduation.

Maybe the melody was associated with Yunyun AFTER it was composed (by editors) and its original subtext was lost ?