Iron Golems Spawn everywhere. Ex: water, lava, half slabs, 2.5 block high rooms.

Hello, my name is prodigy gamer2 and I am an Owner of a realm server.  My friend and I created our first current iron farm on our server and we came to notice that the iron farm was spawning as intended but not in the correct place.  Ex: Lava, on the river bed, way down at bedrock in the neg levels.   

 

So I did some testing on Superflat/Creative World and duplicated my results.  That is also where I found out about the lava spawn.

 

My questions are:

1. Is this a intended feature?

2. What are some of unspawnable blocks for iron golems?