Well this is only 2 years later, so I hope you see this in case it has continued in subsequent lambings.
There is a pretty high chance that this is a nutritional problem caused by lack of calcium. I see it in my sheep for last 14 years. I'm a vet and I've consulted vets and pathologists and no one else seems to have really seen it at this age (i.e. newborn) and I can't find any vet literature on it.Though a handful of people have related same situation.
In my sheep it occurs only when they are being fed corn and/or grazing oats. And only occurs in a few % of ewes. It is not genetic (though of course certain bloodlines could well be more predisposed if they have higher growth rates and variations in calcium mobilisation). These feeds both have low levels of calcium compared to phosphorus, as do other cereal grains or forage.
Where I live we have plenty of sunlight and so Vit D not an issue. But in our colder southern areas (I'm in Australia , so southern hemisphere, so south equals colder) vit D could be implicated, as when dark, cloudy short daylengths, sheep have less vit D, and Vit D is needed for Ca absorption. This could also be a problem where sheep are shedded.
Of course my problem has come about because I either didn't add limestone in with their supplement, or they didn't eat it, but I've got better at it now, so eliminating problem - see solutions below.
There is plenty literature showing a condition called bent leg, from same cause, but is always in older growing lambs.
I firmly believe that in the newborns, it is simply a transferred deficiency from the ewe. She doesn't show any problem as her bones are mature (though her bones are probably more brittle and susceptible to breakage) and she mobilises calcium out of them, but is still not getting enough. Some ewes manage on the deficient diet, but others don't have enough reserves or are less able to mobilise it back out of the bones and thus the foetus suffers - it develops rickets in the last fast growth stage of pregnancy. Then once they are born, the udder is obviously more efficient at concentrating calcium from the blood, than is the foetus in the uterus. So the lamb has a higher Ca milk (though could still be lower in Ca than a non-deficient ewe). This diet enables the rapidly growing lamb to recover and pack Ca into those reshaping growing bones.
I have had ones far worse than your pic and almost every one recovers in 3 days to 3 weeks. I have had only a couple not recover and my guess is that the ewe was just too deficient and milk not good enough (especially with twins). Had I hand fed the lambs I bet they would have recovered. The severely affected ones start out with their knee at right angles and the top of leg at 90 degrees to body and out forward mostly and walk like a crab. Sometimes the feet are knuckled over.
This is definitely not contracted tendons. Or akabane. Or spider syndrome.
Last year we had to radically change our feeding due to severe drought. My yearling sheep ate a diet of almost total corn and grazing oats. Trouble is I stopped the Calcium when I stopped the corn, (As I had never had problems with older sheep on oats) and 2 months on we had the classic bent leg in these fast growing sheep. Back on the limestone, which they wolfed down, even without salt, they improved dramatically after 3 months, though some did not straighten completely. Unfortunately it's taken me some years to understand that they will eat what looks like unpalatable dry white powder when they need it. For a while I had too much salt in it (at about 50%, which also would have restricted their intake). But also don't be fooled into thinking they can be trusted to eat all minerals free choice with no problem. Some really are very unpalatable despite them needing them, and others are highly toxic (e.g. copper) and too risky.
If you are feeding cereal grains wheat, oats, barley etc, or corn. Provide a Calcium supplement too. Simply purchase salt from your rural supply store (I prefer "flossy fine" - this is just finely crushed - compared to coarse rock salt). Also purchase finely ground limestone (Calcium carbonate = chalk). In a container add limestone and up then up to 10% salt. Let the sheep or goats eat it free choice. (if you only had a handful of sheep you could buy salt from the supermarket and blackboard chalk and crush it up. - still lots cheaper than buying proprietary Ca supplements. Or of course you could do a trip to Dover - the cliffs are just chalk.)
(Note beans and pea type grains do not have this problem)
If grazing on oats: same as above.
If grazing on "winter wheats" same as above but also need to add magnesium - I don't know what forms you have over there, you'd need to ask your supplier. In Oz we use equal amounts limestone to "causmag" - some sort of magenesium formulation.
If darkness and lack of sunlight is also a problem, also give 6 monthly Vit D injections (Alone or maybe in a Vit A, D and E combo).
Affected lambs should receive calcium rich milk. As such, the natural mother should be considered suspect at providing this and additional calcium is probably warranted. However, the milk is probably OK if the ewe's diet is supplemented straightaway and she maintains high blood calcium levels, as milk is made in real time based on what's circulating in the blood.