The way its supposed to work is that if you get the buff every week it extends the timer to last another week, while also increasing the effectiveness. Buying the buff on a different character will however NOT extend the timer and NOT increase the effectiveness. The buff can only be purchased on a single character each week, which encourages you to only play a single class. Making the buff account wide should fix these issues.
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Lost all my stacks too and it sucks tbh. It's something you should think about before implementing stuff like this.