There was a case report of a man who had hair loss only affecting his sideburns. He was found to have frontal fibrosing alopecia (FFA), which is a serious form of permanent hair loss that can progressively spread over the entire frontal half of the scalp. Here is a link to the article: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4502478/.
Assuming that you do have a genuine case of hair loss that is currently only affecting your sideburn area (it is difficult to be sure from the photo you provided), I recommend getting it checked out. It is unlikely that you have FFA or anything similar, but it is best to go and see a dermatologist just to rule out FFA or any other serious form of hair loss.
