When closed hauled above 12 kts the leeward back stay becomes very loose while the windward backstay becomes very tight.
I assume this suggests the top of the mast is falling to leeward as the load increases.
Being a fractional rig the top section of the mast is unsupported above the forestay, see first photo below
I found a photo of the same design yacht with two additional spreaders and diamond stay up to the top of the mast.
I beleive this yacht was used for racing rather than cruising. Note the yacht has a reef in the main.
I assume the diamond stay would prevent the mast from falling to leeward?
What are the performance benefits in reducing the mast falling to leeward?
Seeing one of the backstays flopping about is concerning, is there an issue with shock loads on the backstays while tacking?
Perhaps I need to reef the main earlier to reduce the sail area in the unsupported section of the mast?
The lee cap shroud may be becoming loose because the mast is bending aft, which it's meant to do to flatten the main. To be doing it in 12 knots sounds makes it sound as if the rig tension is a bit loose, although I've never sailed a Robbo 950.
The diamonds on the lower boat are directly athwartships and may be there just to support a masthead spinnaker. This is may be a job for a rigger if you want peace of mind; it may just need some tuning. Plenty of much bigger boats (Farr 11.6, Northshore 38, etc) use single swept spreaders and their sticks seem to be bulletproof.
I'd take some good looks sighting up the mast under load; maybe snap a pic so you can show it to a rigger.
It would take a small amount of bend for the leeward backstay to seem quite slack.
I really don't think it's an issue. If you're worried about it, convert to a single backstay with a bridle at the bottom then all will be good! 
MB leeward shroud goes noticeably slack in anything over 15-18 kts. It is normal. The mast is being pushed hard and will take up all the give on the windward shroud resulting in a slack leeward shroud.
If you have the rig so tight there is no give in stronger winds you have probably over tensioned it.
Having said that, if it is very slack in 12 kts of relative breeze have a rigger take a look just to make sure the tension is right.
+1 Chris.
My understanding is that the fractional rig is intended to allow the forestay to always stay tight helping jib do good upwind while letting the mast bend a bit at the top to flatten the sail where it can be really needed because of not only higher wind speed but wind shear too. ie. The wind speed at the top of the mast is faster than the wind speed at deck level. So the top of the mast is allowed to bend to flatten the top of the sail to reduce heeling.
Thanks for your comments.
The rigging was replaced 2 years ago. I got the rigger back last year to adjust the cap shroud tension as the leeward shroud would become very slack (floppy) when close hauled. I have since tighten the backstays a touch thinking they have been loose also.
As Jethrow said it wouldn't take much movement at the masthead to cause the leeward backstay to slacken. I guess it's only noticeable because I have twin backstays.
