Mobs are spawning on top of the bottom half of slabs and on top of stairs.
Literally can not be more simple of an issue than that.
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The trapdoor thing is normal. They changed the creeper's hitbox to 1.8 blocks so it can spawn in that space to catch up with Java. Except they messed up and made the same change to all 2 block tall mobs. So, on one hand, yes, this is a bug. On the other hand, it's a result of adding an intended feature

Okay I see that, but came here to add that cats are now spawning inside glass blocks. Does this happen if there is a full block below the glass block, giving the appearance of spawning inside glass?

Or what about pillagers & illagers spawning on a block that has fence posts on it? Or them spawning on half slabs? There's not a single block or item in the game that currently blocks mob spawning and that is a massive issue.

Confirmed mobs spawning on TOP of glass blocks. There is no preventable mob spawning.

I'll put it at a 0% chance this issue is fixed and will be passed off as "an intended game feature".

confirmed that we can only use scaffolding currently to prevent pillager spawning below the raid farm where you stand. Pillagers able to spawn on top of blocks that should normally prevent spawning, ie. glass, slabs, etc.

Thank you for your report!
We're actually already tracking this issue in MCPE-40783, so I've resolved and linked this ticket as a duplicate.
That ticket has already been resolved as Fixed. The fix will arrive in the next version or is already included in the latest development version of the game, you can check the Fix Version/s field in that ticket to learn more.
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AND also spawning in a 1x3 space MADE ONLY OF TRAP DOORS!! This is absolute nonsense. Bedrock edition is a complete joke.