I'm currently doing a game with Broken Lords and i have cleaned a complete map of it's neutral faction in the middle of an eclipse, using the Burning soul option.

Nothing have been counted :/

The achievement is still at 0/8000

I found what caused the bug.
I did a test, and the account updated when I played a game with the basic Broken Lords faction against basic AI.
In my earlier game played which lead to create this thread, it didn't count because I played a Custom broken lord faction.

 So you devs would be either: 

-Specify in the success that this must be done with the standard Lords. 

Or 

-Modify the success count to allow all fallen lord factions, including customs.

Unfortunately, I don't think we will be able to change achievements (apart of the fact we are not working in the game, we have some characters limitations etc).

As a rule of thumb, all achievements tend to be with "vanilla game", and yes, you can consider creating a faction vanilla, but in terms of the game, it is custom, is no longer "X faction"