The setup for this bug is that the faction quest sent me to infiltrate (to level 3) a city called Sulin. I haven't made it to that region yet, but have a hero infiltrating another city entirely. As soon as that hero reaches level 3 infiltration, the quest shows as complete.  I've attached the save game where the bug occurs on the next turn.

Forgotten - Quest Bug on Next Turn.zip

Thanks for your report!

I added it in our bug database. We will try to fix it as soon as possible!

Thanks again, cheers~!

Mel