Mobs can spawn inside tree leaves naturally across the world or when the player spawns a tree next to the mobs.
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duplicates 1
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I think this is intended behavior. Basically mob spawning requires a solid block to spawn on, with an air block above that, and no solid blocks in their hitbox. A few non-air non-solid blocks are OK for their feet; grass, scaffolding, torches, etc. Mob heads can spawn inside any non-solid block AFAIK. At least according to the wiki.
Certain mob farms rely on the mechanic of mobs spawning with their head in a glass block; it pushes them to the south-east and, usually, into the kill chamber. "fixing" that would nerf some awesome farms.
So IMO this is a feature request that should not be accepted.
As for spawning the tree next to the mob that's completely different; the mob isn't spawning so it's rules aren't checked. But again I suspect this is intended behavior. Trees COULD check for mobs but that would be a feature request. And what would happen? The mob gets pushed out? Or the tree fails to spawn? Or the affected leaf blocks break? There's no existing mechanic to handle any of those AFAIK. Trees check for blocks, but not mobs, by design.