Nether biomes generate on old worlds when a lot of places have fires around it

If nether biomes generate on old worlds that are created on 1.14 it corrupts the world and it breaks the game, how could nether biomes generate on old worlds? Theres still fires on these biomes, it generates random structures which it happened on 1.16

Steps to Reproduce:
1. If your world on created on 1.14, use the nether portal

2. enter the nether

3. Find places you didn't explore 

Observed Results: Nether biomes generate on old worlds

Expected Results:Nether biomes shouldn't generate on old worlds