User blog comment:B214/Chapter 504: Rift/@comment-5303512-20161001162856/@comment-5303512-20161001164955

It could be that when every he flames are not present or hit by a Ice Devil Slayer, that change doesn't show or disappears, plus there are some instances where there were barely any flames or dark streams covering his arms to show some sort of creature like figures.

As for END awakening thing, I thought the theory was that there was a something related to END that will happen to Natsu, his old memories, another personality, a separate entity, curse powers, etc.?

For all we know what happened when he moved through Time he was still himself before he succumbed to his current self.