Post surgery alopecia

Hi, My 8yr old First gen male (neutered) labradoodle had surgery 8 months ago to his left hind leg for a ruptured cruciate. At the time, his leg and up onto his back was shaved for the procedure. The TPLO was uncomplicated and he recovered well. However, 8 months on, he has a patch of complete baldness on his back the base of his tail (10cm x 10cm) while the rest of his fur has regrown normally. The skin appears healthy, he is not scratching, biting or licking the area and overall he is unconcerned by it. We have discussed with the vet who did the surgery but as an ortho specialist, declined to do any further investigation. 2 months ago he ruptured the cruciate on the other hind leg and had the same procedure. This time they did not shave him quite so close to the skin and already the hair regrowth is looking normal. He is otherwise fit and well with no other health issues. Is there anything that we should be checking? Will the hair ever regrow? Is there anything we can do to encourage regrowth? thanks, Kristen