Hi Tiffany. How old is your hamster? It's normal for them to start losing some fur as they get older, but the best remedy for healthy fur and regrowth is a pinch of linseeds (flax seeds) daily. It really does work wonders. They end up with a lovely thick glossy coat and fur does regrow. It's also safe, healthy and hamsters like the seeds and lick them up. A bag isn't expensive and lasts a very long time. I put a pinch on a separate little dish - separate from the food.
I'm a bit concerned that your vet is giving medication for mites. That can be quite toxic to hamsters and should only be given if mites are definitely confirmed via a skin scraping (which is looked at under a microscope). Did your vet do a skin scraping?
Also linseeds are cheaper
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Sorry I just saw he is 18 months old - which is classed as elderly. I would suspect this is part of the ageing process although also check to see if there's anything he could be rubbing his face and neck against regularly. If they try and dig under a house for example and it's a small gap underneath or if a house door is not quite big enough. Also does he bar chew?
There are a number of reasons for fur loss, not just mites. I think it sounds like an unusual part of the body for mites. Causes can be ageing, some kind of disease, or even malnutrition if the food balance isn't right. That is where Linseeds would help. One way they can get malnutrition as they get older is if they have toothache in their back teeth and aren't chewing their hard food properly. Or if the front bottom teeth have got too long or got broken. Have you checked his teeth recently at the front?
If it's some kind of medical condition then there isn't much you can do at his age and as long as he seems healthy and enjoying life I wouldn't worry.