Copying a world disables achievements in the original world

I created a world called "hardcore kinda," and played it for a while. To build my base I wanted to use dark oak. I made a copy of the world so I wouldn't have to waste time writing the seed down and making a new world, so I could look for dark oak in creative mode. Once I went back to the original world, I noticed that it said "achievements cannot be earned on this world".

I never activated cheats on the original world, and I created a new world in survival, closed it out immediately, and made a copy of that world, mostly to see if I had missed a pop up that said "if you make a copy of this world, you will no longer be able to earn achievements in this world"or something like that. However, once I created the copy of the test world ("My world" and "Copy of My world"), and set the copy to creative mode, the original test world could still earn achievements.

I don't know if it's possible, but if you could manage to turn achievements on in that world, that would be great.

Edit: I was also using a custom texture pack and had a custom skin on