zombie spawns after you attack zombified piglin

zombie spawns in the nether after you attack zombified piglins

What I expected to happen was...:

Zombie probably shouldn't spawn in the nether after I attacked the zombified piglins

What actually happened was...:
A random zombie keeps spawning after I attack zombified piglins

Steps to Reproduce:
1.  attack a hoard of zombified piglins in a large space at a high y level. 
2. wait about 30 seconds after this happened 
3. a normal zombie should appear for some reason.