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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Agreed. It wasn't even clear that it was 'Per Character' I myself didn't figure it out until I messed up by purchasing the favour on my alt character. I went back to my main character after I realised but it won't allow you to get the favour again so no boost. Really bad form Bungie.