Specifically they can spawn inside of certain transparent blocks, such as Iron Bars. Possibly a duplicate of **MC-140145 but I had trouble understanding what he was saying.
All other spawn conditions must be met, but the game misses the fact that transparent blocks are not air.
Environment
Java version is up to date, 1.8.0_171-b11. Windows 10, multi-core laptop.
Fixed, with the exception of Iron Golems...