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 have the same issue as OP. My sims are married BFF's and their romance and friendship meters are maxed out but they still are only lovebirds and not soulmates. Cannot fulfill the requirement to become soulmates.