I think the game only checks to see if Romantic Satisfaction needs to be triggered when the romantic relationship first goes above a certain threshold. So for instance if the threshold was a romantic bar of 25 and your couple had a maxed relationship when you installed Lovestruck, it won't trigger because the game isn't seeing the relationship go from like 23->26.
You could probably do a work-around by instead of actually getting them divorced, you could try to cheat their romantic bar down to like 10 and then back to 100. There was a thread on Reddit with a similar issue where half of a couple didn't get the "Fell in Love" milestone because she wasn't in the active household when they met the threshold, and he was able to fix it that way.
