When grinding some zombiefied piglin for about half an hour I noticed a normal zombie had spawned, I retried this once again later in a zombified piglin farm and zombie did indeed appear, it happened on the nether ceiling about 2000 blocks away from any portal. It has happened to my SO as well so it's not just me playing with optifine and such.

If it catches more attention or happen more than the 5 times it already has I will try to get some more info out of it to post.