Occasionally, when I have been playing in the nether, zombies have spawned nearby in two unique instances, both being in 1.16.1.

In one case, when I was fighting a group of piglins, I accidentally hit and angered the nearby zombified piglins. AFter getting to a spot where they could not reach me, and killing most of them, an armored zombie also joined their mob. (While in nether wastes biome)

The second case was at an AFK gold farm above the nether roof, also in a nether wastes biome. After 16 hours of afk, upon my return there were 3 zombies outside of the spawnable area for zombified piglins. The z. piglins are permanantly angry by design of the farm, making it seem as if zombies can spawn as reinforcements for z. piglin fights in the nether, which is presumably a bug. I am aware that this issue was reported seperately in pre-release 5 I believe, but that issue stated resolved, and this occurs still in my world.