Happened to me since 0.9.0 i think, but got worse in 0.15.
Basically, damn mobs spawn everywhere, including places they are not supposed to spawn (Slabs on blocks, etc)
My map is very well lit to prevent spawning, and ive checked it with a special shaders, which reveal areas mob can spawn (light level thing)..
The screenshots are made today in 0.15.1
Check out creeper spawned on occupied by glass panel block
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Thank you for your report!
However, this issue has been closed as a Duplicate of MCPE-14467.
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