This is most unusual. I have done two playthroughs of Driftmoon using cheats since, both on Guardian difficulty. The first time I didn't get the achievement, the second time I did. The only notable difference between the two runs is that in the first one I had saved and reloaded, only from the autosave. In the second I did it in one sitting, with no saves used.
Perhaps some flag for the achievement doesn't get overwritten when there's an autosave over a previous file? Not sure, but there's something strange going on here. And that's definitely something that both of my Guardian playthroughs have shared, reloading from the autosave slot which previously held a Warlord save.
I love the game though, it's been a joy to play through all three of my regular playthroughs.