I had a sim with the Soulmate aspiration and got the goal "Get married to BFF". By the time I got the goal, I was in a relationship with Soulmate status. We got Engaged through the conversation option...
I too am experiencing this issue, only I actually am BFF with my spouse, it just is not recognizing that fact. We were BFF and Soulmates before we got married, but the aspiration goal will not pop.