I beat Decidiwi for the fifth time a while ago. I'm up to eleven wins now, five since the last battle reset.

And it gave me the 5x achievement again, complete with the achievement prize. I don't know if the achievement was still green before the fifth win, but something's definitely up.

EDIT: Soemthing is definitely up.

I am up to thirteen from five, but the overall count is still at eleven, and it should be at least nineteen -- assuming that I wasn't at eleven from before the clearing and it just didn't count the next five? ...I suppose that might be a cache issue? We'll see what happens when I reach twenty-five...

EDIT:

Up to twenty for this time around, and the count still says eleven. Viewed my statistics page for good measure, and the count's fine there. I should have a total of at least 26. I'm gonna give it an hour or two to do its thing -- sometimes the wardrobe achievements need that -- and then I'll check back.

EDIT: I'm a derp. Forgot to check eventide phoenix achievements before starting to fight that guy...but it looks like the same issue is occurring. 5x achievement awarded, and no increase in progress to the 25x achievement.