vets have done every test under the sun but can't explain it
Have any of these vets been veterinary dermatology specialists?
I started researching thru some medical and other reliable sites for some clues.
There is a common thread in these sites when describing alopecia x, which is known now by several names, and non-inflammatory alopecia. These articles and papers may not all be terribly current in diagnostic practices and terminology but the physical description is very similar.
Adult-onset Growth Hormone-Responsive Alopecia
A
disorder of uncertain and questioned pathogenesis. High frequency in the Pomeranian, Chow Chow, Keeshond, and
Poodle. Onset of coat problems
early in adulthood (1-3 years). Males over represented. Coat changes seen include coat color change, loss of primary but retention of secondary hairs, and
hair loss in the collar region, thighs and ventrum. The alopecia becomes
more complete and widespread with advancing time. Exposed skin hyper-pigments quickly and markedly. Beyond the skin changes, the dogs are otherwise normal. Some dogs appear small in stature but are at the low end of the breed standard.
Affected individuals show minimal growth hormone response to the administration of xylazine and many regrow hair when growth hormone is administered. Since there are no specific growth hormone receptors in the skin, not all dogs respond to treatment, and there is a striking breed overlap with the adrenal hyperplasia-like syndrome, the growth hormone "deficiency" may just be part of the adrenal dysfunction. This is supported when the dog regrows hair with treatment of the adrenal disease.
Sex Hormone Dermatoses - WSAVA2004 - VIN
Hair Loss with No Skin Inflammation
- Non-inflammatory alopecia is a group of uncommon skin disorders, characterized by hair loss that is associated with an abnormal hair growth/shed cycle
- Hormonal and non-hormonal diseases can be associated with non-inflammatory hair loss (alopecia)
- Alopecia X is a non-inflammatory alopecia related to an abnormal hair growth/shed cycle; it has been called by many names previously, including “growth hormone-responsive alopecia,” “castration-responsive alopecia,” and “adrenal hyperplasia-like syndrome”
Breed Predilections
Alopecia X—miniature poodle
Signs/Observed Changes in The Pet
Hair loss (alopecia)—usually generalized and bilaterally symmetrical; involves the trunk, along the sides of the body (known as “truncal alopecia”) and
spares the head and lower legs;
Pet Health Article: Hair Loss with No Skin Inflammation (tricountyah.com)
Non-inflammatory alopecia
The causes of non-inflammatory alopecia include hormonal diseases (such as hypothyroidism, hyperadrenocorticism or Cushing‘s syndrome, hyperestrogenism, and Sertoli cell tumour) and hair follicle dystrophy/dysplasias (dilute colour alopecia) and alopecia X.
Non-inflammatory alopecia usually appears symmetrical, in the absence of other skin lesions and without pruritus.
Alopecia X
The pathogenesis of alopecia X has not yet been fully established. Predisposed breeds are Pomeranian, Chow Chow, Siberian Husky, Alaskan Malamute and Samoyed.
Symmetrical non-inflammatory alopecia of the neck, caudal thighs and trunk are the dermatological lesions that occur without systemic clinical signs. The alopecia can appear both before and after castration.
LK_Alopecia in dogs _EN_BattLab.indd
This article goes over several, including color dilution alopecia which may also show the same pattern of hair loss.
Focal, non-inflammatory alopecia: A diagnostic, treatment challenge (dvm360.com)
Another
Differential diagnosis of non-inflammatory alopecia in dogs (laboklin.com)
One more
Alopecia X in Dogs (Hair Cycle Arrest) - MedVet
The common thread in these is the symmetrical hair loss which may spare the head and lower legs from the loss.
I'd look to a derm specialist who's dealt with alopecia x and non-inflammatory alopecia.
I wonder if Mayla’s story might be helpful for you: 
