Thanks for the responses!

Shouldn't be a married segments issue since these were all standalone segments (with a >24h stopover inbetween) and the one F -> J connection is showing availability online for a J -> J connection as well as a standalone booking.

So unless SQ has implemented some "we'll only sell this segment as a standalone award or as part of an int'l J award, but not as part of an int'l F award" nonsense, married segments doesn't seem like the explanation.