Honestly? After talking to dozens and dozens of post-op bariatric surgery patients, it depends more on your genetics and your luck than anything. And I hate to say that. It's kind of like the loose skin issue. I wish I had a magical trick for the huge number of people that have asked me about my lack of hair loss and loose skin, but really - I got my protein in, I drank my water, I used bath and body works lotion a few times a week.
170 lbs lost, never lost any hair, barely any loose skin. I have no idea how, or why - neither does anyone else take your biotin, get your protein, drink your water - you could try to find some biotin shampoo! But honestly, I don't know if it makes that much difference. It seems that some people just lose the hair, and some don't. Good news? It will grow back!