When Trixie returns home, she instantly thinks about all the times he's been there for her. So, Trixie goes to the main hall of the mansion, sits down on the piano, and plays "Worth A Chance", by Steve Conte. After she finishes, she looks at a picture of Timmy and says, "I never realized how much I meant to you until you left me. Aishiteru...Chiimy-kun, please come back safely."
Let's hope then the writers didn't forget that detail unless some writers did that with purpose to ignore "Da'Rules" because they might have some bias toward another pairing we know just to please Nintendomaximus
Btw, it could be interesting to imagine a similar scene using other cartoons characters like Todd-Maurecia (Wayside), Danny-Sam Manson(Danny Phantom) or Duncan-Courtney (Total Drama Island)