After updating to Version 1.17.1 I traveled to an undiscovered chunk on my map and put down a nether portal. When I went through the portal all of the nether was covered in stone and no mobs are spawning. I did come across a nether castle but even the bottom of the nether has no lava. When I then traveled through the nether to my previously rendered nether chunks and those were all still the same but I am not seeing mobs spawn anywhere.