Also confirmed; and I know what it's doing wrong (but not why) -- for any achievements that are "goal over time" and not specifically in one game, it will only count the first game you play as that Demigod.
This goes for the "win on all maps," "win X games," "kill X Demigods," and also such things as TB's "Deal X damage with Rain of Ice" and Rook's "Raise 100 towers using Tower of Power."
This is most noticable by checking random people's achievements on Pantheon and seeing that everyone is stuck at 12% for "win a game on every map as [insert DG here]." 12% (really 12.5%) is 1/8.
So if you get, say, 10% of the achievement done in your first game as that Demigod, it will remain stuck at 10% forevermore. But if you actually fulfill the requirements in the very first game it will recognize it. For instance if you've never played Rook then you play a game as him and raise 100 towers within that game, you'll get the achievement. But if you raise 50 then it'll stay stuck at 50% even after subsequent games.
Not sure what happens if you partially fulfill it, then get the full amount required in a later game.
This bug does not affect achievements that can only be obtained during the course of one game, such as "Win a game as ____" or "Do X damage with [attack] in a single game." It's only for those that save your progress over multiple games.