Sunburn on day 13 after HT. Could I have ruined the grafts? Or is this normal shedding that's been accelerated? (photos)

This sounds like typical shedding at the two week mark. Expect all of the transplanted hair to continue to shed, and the permanent growth to come in at around the three month mark.

After 2 weeks, you may be experiencing shedding of the transplanted hairs, which is a normal process.

Thank you for your question. You should always wear a hat when you are outside because you can still get burned on a cloudy day - the sun's UV rays are very strong. Please follow the recovery instructions and consider consulting with your board certified facial plastic surgeon and medical team to ensure you are healing effectively.  
Best wishes,

Great question. I understand your concern but you don’t have anything to worry about. While its not a great idea to get sunburned, at the 2 week mark it will not hurt your grafts. The shedding you’re seeing is normal. In the future wear a hat while outdoors to avoid any potential problems.

Walking on a cloudy day in and got a sunburn on your scalp could not have ruined the grafts. Grafts take well  after 10-15 days after surgery. Transplanted graft’s hair starts to shedding in the first week of surgery and may go on untill 4-6 th weeks after surgery. İt is not surprising to lost approx. % 70-80 of all transplanted hair shafts in first month when you are washing, gently rubbing  or touching the head. İt is shock loss and not permanent.

Two weeks after the procedure the grafts are already secure and sun burn will not effect them. The shedding of transplanted hair is normal at this stage, they will start to grow in after a few months.

By this time the grafts are secure and a sunburn may only retard full healing a bit, but generally will not impact your overall graft growth