Put the summary of the bug you're having here

What I expected to happen was...:
Ancient Debris to generate as normal

What actually happened was...:
Ancient Debris did not appear to generate as normal

Steps to Reproduce:
1. Generate a world in this snapshot and search for Debris in the nether.

2. Use /fill to remove netherrack (although I observed this through survival game play, after mining normally at y=15 for a significant amount of time and finding 0 debris)