Hello.
I had the same "problem". But then I was thinking. I don't know, but english is not my native language, but I think this goal was "have HAD 5 BFFs". And this means it was in past, and not in present. I mean if we look the grammar. So I think it is not a Bug. In real life you have just one REAL best friend at the same time too. :) So the goal is not to have at same time 5 BFF but simple had them. You don't need to fight with them, just the time goes and they will drop it. Then you can ask an other to be BFF.
Oh and choose not a family member, because in my oppinion our relatives/family is always a BFF without "asking". You can't choose them, you get them as you born. :) While friends comes and goes as you live your life and as you change/grow. Or if you want to be with one of your family member, then be with them the 5th and last one. So after the goal it will stay.
Please no offense I was just telling a logic in this "bug".
Take care and happy simming! :)