If it's a glitch it might be best to refrain from using the coupons for the time being?

The coupons themselves don't seem to be a glitch...just the part where you can get exponentially more of them. And said exponential coupons have been yanked, according to reports on the Steam forums--but at least one or two coupons have stayed behind, which suggests the actual handing out of the coupons doesn't seem to be a glitch.
As near as people can guess, the number and type of Ubisoft coupons you get seem to relate to the games you already own. For example, I don't own any of the Might & Magic / Heroes games, so I didn't get any of those coupons. But I do own Assassin's Creed 1 and 2 through Steam, so I got two coupons for AC:R. Seems to be some sort of ninja loyalty reward.
They're probably safe to use, but if you're worried about it, wait a few days and see what happens. They don't expire until April 1st anyway.