so i played with the new faction. had the quest to dystroy a fleet inorder to assimilate the minor faction.
so i dystroyed them and then i got failed in the quest?!, and the minor faction just disappear?

i had a umbral choir AI, but i dont think they assimilate the minor before me... I had try and test it again and again. when i compliet the quest. the minor just disappear

the fleet is on the accademy if it help

BAG_01.sav
BAG_02.sav

I just checked your save game: The Umbral Choir was hunting the same fleet as you for the same assimilation quest, but had better relations with the minor faction than you did. I believe this is working as intended, but I would have to double-check with the Game Designers to be sure.

EDIT: GD's confirm you should have gotten the assimilation. I'll pass your save along with the bug report.