Alli

I was under the impression that this was intentional behaviour for Zombies. They are supposed to be "stupid and impulsive" so they will walk directly through the fire to pursue an enemy, without regard for their own safety.

Mobs that are considered "smarter" should try to pathfind around the fire.

Well, the Illagers do have their "Fake End Portal" rooms hidden in the Woodland Mansions. Perhaps this is the reason they have the capability to raid the End, but not the Nether...