Mobs are spawning on transparent blocks (tested blocks - standard glass block && cobblestone half slab [lower half]). Mobs confirmed to spawn on these blocks: Iron Golem, Skeleton Archer, Zombie, and Creeper.
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Same happened to me when I just made my Iron Golem farm and one Iron Golem literally spawned inside glass. Have a look.
There is no open gaps. It is all glass surrounding the villager.
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I can confirm I have replicated this in my Realm. Mobs spawning on glass blocks. They are in the ocean and there aren't any solid blocks for at least 100 blocks.