I've found something that is most likely related to the issue at hand.
There's a new mixer_social_NPC_greetings in the interactions, which I assume is used when two NPC Sims are meeting, it's available autonomously but not to the player.
This new interaction is granting a new Loot_Relationship_NPC_Initial_Social which have different potential outcomes, like setting the friendship relationship at various levels (-50, 40) and the romantic one at 40. So it looks like it would match what has been reported.
It's all in the XML, so assuming there's not another mechanism elsewhere too, it should be easily moddable, by editing the Loot_Relationship_NPC_Initial_Social so I can give it a try.
