My Hardcore world switched to survival

I logged into my hardcore world for the second time today, (earlier it was still in hardcore mode). I was mining for more netherite when I looked at my hearts and realized that they didn't have the dark indent in them like the hardcore hearts. I checked the game difficulty and it was no longer locked. When loading up the world it still says "Hardcore Mode!" under the world. This is extremely frustrating as I had been doing extremely well in this particular world.