My nether turned into another version of the overworld that is always dark.

My nether turned into another version of the overworld that is always dark and it has just about everything i did in the overworld with a few exceptions, probably due to when the game last saved.

What I expected to happen was...:
to end up in the nether.

What actually happened was...:
something way scarier, as wither skeletons where spawning everywhere you, turn plus all of the overworld hostile mobs, with what seemed to be no restrictions, except the areas i had previously lit

Steps to Reproduce:
1. I kept on trying to replicate the issue, but to no avail.
2. All i do know is that i was on hard, I was trying to kill a straggler zombie pigman that escaped the nether, but i ended up hitting him in the portal so i decided to follow. The nether didn't seem to load in properly then i herd the pigman die and found myself in this hybrid dimension.
3. ...