I am also having a trouble with the Soulmate aspiration's Achieve soulmate relationship with spouse requirement.
In other save files, I've never even noticed this requirement; it just seemed to complete itself while I was trying to ask to be girl/boyfriend, propose, move in, and so forth.
Today, though, I have couple with max Friendship and Romance but their relationship won't change from Sweethearts to Soulmates, no matter what I have them do.
This is just a practice file, so I'm not so fussed but I'd still like to figure it out.
If I use the aspirations.complete cheat, Tier IV has the requirement Do 50 romantic interactions while Soulmates, so the issue persists.
I note that OP is counting the romantic interactions herself.
I tried clearing the cache files, deletiing the Origin App data (Local/Roaming), repairing the game, and traveling to different lots.
Why, I wonder, does my game glitch in one save file and not others?