As the title says, I've been playing on a hardcore world for a little while now, and after exploring the end for a little bit I realized my hearts changed to the default texture and I was able to change my difficulty settings which I'm not usually able to obviously being in hardcore. When in the world selection menu, it still displayed "Hardcore Mode!" in all red underneath the name. I've locked my game in hard again but it still displays the default heart texture. Below are screenshots that I took that shows the change in hearts (I don't know specifically when the hearts changed). 

Edit: I made it back to the overworld and my hearts are back to normal (hardcore texture) after leaving the game and joining again so I believe it's an End exclusive bug