according to minecraft wiki spawn mechanics you need at last 2 blocks of headroom above an opaque block to spawn a mob , the last two snapshots broke the spawn mechanic requirements for hostile mobs.
here's an example that tall mobs and spiders can somehow spawn on a piston with 1 airblock above. and get pushed on the glasblock.
[media]if i close the gap between both quartzblocks, mobs can't spawn anymore.
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this is just an simple example. if you want to reproduce it, just make postons working (pushing glasblock around)and let the game running. i don't know if the mobs spawn on the piston or on the extending piston head. abut somehow they are spawning.
[media]few minutes later a mob spawned on the piston or the piston head and gets pushed through the glasblock.
made an example vid now.
I can confirm similar problems, they are able to spawn in certain transparent blocks; in my case Iron Bars.
cannot reproduce