With recent changes to glass (and stained glass) from non-solid to solid to (almost) non-solid again, mobs now spawn on it. May be related to MC-139394.
Edit: Further tests showed monsters also spawning on most transparent blocks, as long as they provide a full surface. Tested Glowstone and Sea Lanterns in the Nether, since light doesn't affect most Nether monsters.
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Just as a heads up
Make sure not to break the feature of placing redstone ontop of transparent blocks when fixing this bug
Looks intentional, and kinda makes sense, like, why would mobs spawn on stone but not glass with the same shape?
Fry
The title should be changed to something broader like "Monsters spawning on transparent blocks" because they also spawn on Sea Lanterns, Ice, Glowstone, Trapdoors (top) etc. Basically, every transparent block that has a a full surface (i.e. no spawning on stairs).