Then maybe it's mutation of the yeast? This was maybe 3rd or 4th generation. But if it were mutation I'd expect it to have the flavor even after a week.
This is a beer I make all the time, and it has never had any Belgian taste to it before at any time during the life of any previous batch. Fermentation was same as ever, controlled to 66 degrees F in a fridge.
I did turn up the temperature for the last two weeks to try to get the Stout that was fermenting alongside the wheat to drop some gravity points. Maybe that was it...
At any rate, that yeast is gone, and let's hope the IPA comes out clean.