Hello!

I would like to inform you that the said event isn't bugged. The appearance of each item together with the percentage rate of discount is all based on chances or randomness. It is and has been that way for a long time. If you want to minimize the damage to your resources, it is much better to do what Goshi said above but if the item you want to purchase didn't appear even for once with numerous attempts then you should try to reconsider your plans like the number of coupons that would like to spend for buying keys plus the cost of the number of fragments that you need versus the price of your wanted cloth in other events. Do not let your urge to get the item badly to take control of you or else, you will be left with nothing after dealing with RNG. Thank you.