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Not sure what the deal is with OTAs (try to avoid them as much as possible), but the ~$2.3k round trips booked through SQ are on a D fare basis (D15USRLC) so it's an actual Business fare and it shows up as business and all that.

That said I dug a little deeper into the fare rules and it seems like this is a JFK-FRA RT sale fare that somehow allows for a stopover in SIN(!) — which is why when booking on the SQ website one has to book multi-city (e.g. JFK-SIN, SIN-FRA, FRA-JFK) to get a ~$2k price, while JFK-SIN RT prices at ~$4k.

It looks like when loading this sale fare /SIN got appended to the rules: 2 STOPOVERS PERMITTED IN EACH DIRECTION IN LAX/SFO/NYC/HOU/SIN, and oddly enough SQ's routing rules for all their JFK-FRA fares allow a stop in SIN:
/VIA THE ATLANTIC/
MAP CONSTRUCTED LEFT TO RIGHT AND RIGHT TO LEFT
1. NYC-SQ-FRA-SQ-SIN-SQ/MI-FRA
2. NYC-SQ-SIN-SQ/MI-FRA
3. NYC-SQ-FRA

Which is how we end up with a valid ~$2.3k JFK-SIN RT fare (well JFK-FRA with a stop in SIN).

In addition some of the cheaper fares with either JFK-FRA or FRA-JFK in Y or PY seem plausible given how the D15USRLC fare looks like it's combinable (e.g. JFK-FRA in V/W + FRA-SIN-FRA-JFK in D) — and probably explains the first post as well. As for your Kayak screenshot the base fare for D15USRLC is $2230 and V15USRL is $480, so $1448 looks reasonable after you throw in taxes and fees.

Not sure if this falls in the realm of an error fare, and the fare rules haven't been fixed for over a week, but we'll see what SQ does with the bookings I guess (unfortunately I only thought to examine the fare rules >24h after I booked one of these RTs, so can't cancel now and will see what happens)

Also am not sure what the etiquette is regarding this but if I really shouldn't be bringing this to the attention of SQ happy to delete this post as well