I was walking through the lovely new nether biome and I found a massive wall of netherack, where the warped forest biome and nether wastelands connected. I found similar walls on chunk borders in the overworld.

Generated on a server world which was made in 1.15 but only generated the nether in 1.16. The nether access was turned off during 1.15 due to player preference, so the nether was never entered or loaded, and so the DIM-1 folder never deleted.
seed:[-1043313485117864560]
co-ords:
nether: 990, 78, 326 (x,y,z)
overworld: -3770, 64, -3000

Im not 100% if this is a bug but I hope that it helps.