So I'm wondering if anyone knows a solution (or at least a way to avoid a bug in the future) where this quest become uncompletable due to the requirements being satisfied but not triggering the progression of said quest.

The situation I have encountered is as follows:

I destroyed the city after I already had all villages in the region converted, which may be related to the cause of the bug.
Moving out of the region with my units (solution the OP in the technical support forums found) does not trigger the quest progression for me, and neither does having a village destroyed and then reconverted.
This is in a unmodded multiplayer game with all DLC enabled, but I am using a custom cultist faction that still has the auto-raze city on capture trait, so I don't believe any of the selections for traits I made should be causing this bug.

Hello!

Do you think you could upload a save with the issue happening?

I will try to reproduce it at our end but a save is always helpful to find bugs and to fix them :)

Thanks a lot.

Sure. I think this is the save I have closest to the bug.
The Chosen - Turn 108 .zip