Ideas
Had a few of my couples do this too. Will be somewhat long so bear with me trying to pinpoint in the dark.
Some of them achieved soulmate status when they were teens. So the other half tend to be a few days older and then grew up to Young Adult. In that few days (where I turned off aging until some Sims completed University, almost a whole year) their romance bar decays significantly (no longer soulmates, just lovers) while the other stays a teen until their own birthday, after I turned aging back on. So now that the couple are both Young Adults I got their romance bar FULL again, but soulmate is unattainable.
However I did have a different couple where the exact scenario above happens, but they regained their soulmates status JUST FINE. They both prompted me with the "So I found a Ring" phone call in which I tell them to say yes, resulting in them getting engaged: Young Adult x Teen. Is that gonne be a problem?