Check to see if it is under warranty

Was a clear coat ever applied after it was painted?

Unfortunately, it's an all too common problem irregardless of where you live. I see much of it in Ontario. What I recommend is to sand the door down and get rid of any flaking paint. Repaint with a good quality paint preferably with an UV ray inhibitor. Then apply several coats of marine varnish or spar urethane which has it's own UV protection and remains flexible throughout the changing temperatures of the seasons. As well as helps keep the water out.

I hope this helps.