When Killing Zombie piglins in the nether i found that zombies sometimes spawn as reinforcements.

You can easily reproduce this by finding an area with many zombie piglins, then begin to kill some of them in survival to anger all of them, then there is a chance that a normal zombie will spawn to attack you. 

I saw another bug report on this exact issue but it was marked as solved but i still can recreate this bug very easily on my 1.16.3 world.