This must be a feature, as I don't recall a time when it didn't happen, but I don't think it should be! If the controlling Sim speaks to other Townies while on a date or event, fine. But I have taken Sims on dates where, in no time, the couple will be joined by 5 others in conversation, and it just makes it harder to play smoothly. Every chat function takes a {hot} minute to complete, so multiple conversation hijackers on a timed and rated event, really messes things up.
And I'm really loving planned, goal-oriented events lately. I wasn't into them at first, but they're really beneficial for a number of reasons! Anything to decrease snags when you're already planning a lot is important! And this isn't the first time I've rediscovered old features for new gameplay. It happens all the time. It's why I love this game!