No but have you seen such a study for all the signs normally used by neurologists to make diagnoses? Neurologists learn the validity of these signs as trainees doing full neurological examinations up to ten times a day, making a clinical diagnosis and the seeing if it is confirmed by imaging and neurophysiology etc.
I don't see any reason to doubt that the signs show an absence of a local peripheral lesion - which is the point of the operational FND diagnosis. I don't know how many patients I have examined but I did six months as a full time trainee neurologist and it must have been hundreds. In other contexts the neuro exams I have done must run into many thousands. Do we have studies confirming that the tests run by mechanics to distinguish a blocked fuel feed from a dirty plug are reliable? Once a mechanic has got 200 engines going again he tends to be pretty reliable.