Also it's probably not something on their end that went wrong, probably more to do with Itch.io's way of storing what was paid for products.
So the devs probably (justifiably so) increased the price and then Itch just goes "We don't store what product you purchased just how much you paid on what projects. So since you paid 5 bucks during a 50% off you get access to everything up to 10 dollars, regardless of how the products price develops."