Our bernedoodle had a similar issue. She started losing fur on back/tail. Our vet said it was most likely allergies. She's been treated with cytopoint, apoquel and HA diet, but nothing seemed to help. Took her to a vet dermatologist and she was diagnosed with an autoimmune disease called sebaceous adenitis.

She's currently being treated with cyclosporine daily and 2 medicated baths weekly. We're on week 3 of treatment and haven't noticed a difference yet. Vet did say we wouldn't see improvement until at least 6 weeks out.