I have the entire loaded area in my realm covered in half slabs, and mobs are still spawning. It’s a big hole all the way down to level 5, covered in half slabs and zombies, skeletons, creepers, etc are all spawning still
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May be related to https://bugs.mojang.com/browse/MCPE-46396

Watched 4 zombies and 3 spiders spawn on lower half slabs next to witch hut on a realm. Had only slabbed two chunks with the entire surrounding area unaltered. Seemes like the hostile mobs prefered to spawn on the lower half slabs

can confirm this is a problem on most recent (as of writing this comment) version. found a creeper and spider on my a fully slabbed/glass area.
there are also spiders in my creeper farm where they shouldn't be able to spawn. have been trying to fix it for most of the day. but it might be related to this issue (using carpets to block spawning)

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We're tracking this issue in MCPE-40783, so this ticket is being resolved and linked as a duplicate.
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Skeleton spawned in my house with no spawn able blocks, all slab floors. Spawn was also only 10 blocks from me. Similar incident some time back with creeper, both times on getting out of bed. Rare but significant issue for me, not consistently reproducible. Using current android build of bedrock.