Feed play's a big part in stiffle lock in young, I have delt with it BUT never delt with one that would lock and NOT unlock. This is a pic of a 2 month old rescue, as you can see he was away from the mare way to soon, and was not being feed anying. 2 months of GOOD mare a foal food, we have never had an issue & he is now 3 years old
We did have a vet over see him with this & the vet said give it time, with being under feed & not moving around like he should of been ( to sick & tierd to be like he should of been)
and he was so right, good food and it was gone.
He would be running and it would lock, it would make him fall.
My vet said feed GOOD Mare & foal feed then weanling feed
&
Walking them back wards will help unluck it.
Walking him un & down a sloped hill will help build his musels to fit the bone.
How was he health when you got her?
What has she been eating since you have had her?
she needs space to book around, once it does un lock I dont think it would be good to keep in a small area ( like a stall ) she needs to move.. I would not have her in with others. If she gets to running with everyone or needs to get away and it locks up it would be unsafe..
Theres hope that this will go away with out surgery & since she in young there is room for growing & that alone can fix it with good food.
Good LUCK & hope it is a easy fix for you & her