There is something odd going on with EK generally I think. Try this one:

I tried to book NCL-DXB-SIN-DXB-LON today and was told on the phone that NCL-DXB had no J seats left so I could do Y on NCL-DXB and then F on DXB-SIN (as it was 'cheaper to do that than go in J from DXB-SIN).

OK, I thought but decided to try from MAN instead of NCL and got the same problem. No J. Only F.

HOWEVER! ExpertFlyer shows J4C4I4 on both the NCL and MAN flights I wanted.

So I then decided to price it up myself online (the reason I hadn't before is that I was on the phone to the agent about cancelling another flight I had booked so thought I'd get them to price out the itinerary instead of doing it online). Sure enough, when I go to do a RT NCL-SIN rather than flying back into London it allows me to price in J all the way for substantially cheaper than doing Y+F or F/F from MAN!

Bizarre.