Did you book online? because I cant seem to book to the 2nd leg online
No, over the phone.
Both legs have to be available separately in I class, and the phone agent has to see both separately available for saver price.(Sometimes SQ reps dont see the same availability as the website, so thats why Im saying that they have to see it also, not enough if you see them separately online.) Once they see both in saver they can piece it together as a Star Alliance mixed award.
Actually at first they couldnt price it together, but then i asked them to first sell in the SQ flight and end the record. Afterwards to add the LH leg and it worked. Priced at 72k.
Good luck!