The optional conversations by the graffiti house and the ice cream truck will only trigger once per PlayStation profile. If you triggered these conversations in a previous playthrough, they will not trigger again when exploring the Suburbs but will still be counted towards activating Ellie’s joke.
To confirm these conversations occurred, check the Suburbs Collectibles activity card. If “Talk to Henry about graffiti” and “Talk to Ellie about ice cream truck” are NOT listed, then they have already been counted in the joke steps and the optional conversation trophy.
In order to get the joke to occur, follow all other steps listed here and wait in the area. If your profile has already collected the rest of the conversations here, all you need for the joke to activate is to hear the kitchen BBQ chat and then to head to the Firefly graffiti at the end of the road.