Jokes aside, I think there was likely a 2 pax PNR waitlisted for proper J (i.e. not an upgrade), so with J1, it couldn't clear, but with J2, it could.

So it's just another thing to consider in these situations.

I still don't think I've ever flown Y (on a multi-cabin aircraft) on a Y FP, nor have I used eUps. But sometimes things can get messy.