/fill does not remove deny blocks under y level -62

Hello, I was building on my world, and I had used the fill command to add deny blocks, which I had later removed using the fill command to turn the deny blocks into air, but when I reloaded the world the next day, the blocks returned to the same place. I tried to destroy all of the deny blocks again, but apon reloading the world, the deny blocks returned (but some of it was destroyed)
I have no idea what is going on about these deny blocks.

Thanks for reading.