Did you resolve this? I'm a bit late to the party, but am surprised by this. We worked out If there are 3 or more seats available on Scoot, the seats go into inventory as SQ codeshare. We actually booked a SQ codeshare SIN-PER (on Scoot) then return on SQ, MEL-SIN. ScootPlus seats map as Premium Economy on SQ though, so I expect this would LHR-SIN be booked into Premium Economy as well.
[nb: This could also be bad advice on my behalf, as we got booted off the Scoot flight anyway and SQ cancelled our ticket (refunded within 4 days of agreeing to cancel ticket). ]