Mob spawning (In the Nether)

So, I've encountered a bug with mob spawning (Specifically in the Nether from what I've seen). Say I have the difficulty set to normal, but I change it to easy. You'd expect mobs to still spawn, right? But this doesn't seem to be the case. Mobs stop spawning altogether, and I can only fix this issue by saving my progress on my world and going back on it.