From what I understand, SQ splits its award flights into 2 inventories, one being the 'only SQ flights' inventory redeemable by Krisflyer miles only, and one being the Star Alliance inventory. The latter is also the availability which you can see when using miles of other Star carriers to book SQ flights. As we all know, this availability is way way lower than what you would get when using Krisflyer miles to book SQ flights. And even worse is that some Star carriers are further blocked from seeing some of this inventory.

The problem with combining an SQ flight with another Star carrier in one award is that SQ classifies this as as 'Star alliance award booking', and will only look at the second inventory of flights even for its own SQ-operated sectors (i.e. extremely low availability).

The SQ flights you have found is probably only available as an SQ (not star alliance) award booking using Krisflyer miles from the first inventory. And what the computer tells the agent is that if you want to book this entire sector, it will price out as two separate award tickets combined together into one itinerary. i.e. SIN-FRA using the SQ award chart, then FRA-DFW using the star alliance award chart. Since SIN-FRA is not available in the 'star alliance inventory', the computer is not able to price the entire thing as a Singapore-USA itinerary.

Hope this helps to explain, but it won't solve your problem.