D: Hey for sure, your brewing its little weird. I don't have to pay anything for brewing, besides to buy the ingredients and cauldron. I will suggest you to ask your MoM or headmaster first. Because they can answer faster. And for brewing, make sure you follow the instructions and requirements. Since it was a site, such occurrences are normal. Hope your brewing went well

Hello! The same thing also happened to me. But I think isn't a bug. WoP is realistic <3 So that is imagine you are a real wizard. You try to brewing potion. Well if you aren't succeed, of course the ingredients won't come back ^^

Good Luck for your potions!