After destroying all of the end crystals - using the last one to deal 10% damage, I died fighting the dragon so I took a break, when I came back the dragon was at full health again. Yesterday I was ready to battle the dragon again. After an hour and getting his health down by 30 - 40 %, I did a save and quit so I could take a 10 minute break. When I came back the dragon was back to full health. 

I will not engage the dragon again until this issue is resolved as it could take 3 - 4 hours of continuous play to defeat the dragon without doing a save and quit and I don't play like that. I need to take breaks.