See the video I uploaded to dropbox to show what is happening. In the video I recorded iron golems being spawned and not falling into the lava, but instead hanging over the lava barely by its hitbox.
https://www.dropbox.com/s/8vgauksxk5jpp0v/Mobs%20not%20falling%20in%20water.mp4?dl=0
Hi there!
This is caused by the iron golem trying to pathfind to the zombie. Try hiding the zombie from the angle of view of the iron golem; that may fix the problem. The iron golem may no longer want to pathfind towards the zombie, and then do nothing and just fall into the lava.