This sporadic and minor bug causes a dupe to get so flattered by sparkles, they can't stop smiling.
This Ren here has the "Balloon Artist" Overjoy response and he's not overjoyed, nor is he sparkle flattered. But he's still smiling! How adorable!
Steps to Reproduce
Overjoy a Sparkle Streaker dupe and let him/her/it Sparkle Flatter another dupe. Sometimes when the "Sparkle Flatter" moodlet ends, the dupe returns back to normal. Other times, they just can't stop smiling.
This issue has not been confirmed by a developer yet.