Not a phantom fare. I booked it last week with a stopover in SIN both directions for £3300. Use expertflyer to look for A class and do AMS-SIN and SIN-Australia separately. The SQ A always tallies with what is bookable through LH website however the LX/LH doesn’t always match when you try piece the itinerary together.

I also did MAN-SYD for £4100 in A class with stops each direction in SIN. If you have expertflyer and loads of time to try different combos with LH website this fare is 100% bookable.

By the way google flights and some OTAs are terrible with these fares due to not recognising LH/LX married sector restrictions. This is why it is best to try yourself and also schedule stops to break up the fare and prevent married segment logic kicking in.