Top of Iron Golem isn't damaged by lava

I have a basic iron farm that spits out one golem every 22-28 seconds. Nothing crazy at all. It has a lava killing mechanism that is set to where the very top of the golem will be in it and die. However the lava doesn't seem to be killing the golem. 

 

I went into a creative mode testing world and it seems that magma still kills golems and I placed lava at the golem's feet and it worked there too. So I'm not sure why on my vanilla survival world the lava at the golem's head isn't doing anything. 

 

I know of the nerfing of op iron farms but my iron farm wasn't breaking records to begin with and I don't think my issue has anything to do with the fix.