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.
The Sims Team
