Monsters will burn underneath half slabs. Despite being protected from sunlight, monsters that can burn in the daylight will burn underneath half slabs, regardless of whether it is a top slab or a bottom slab.
Endermen also take rain damage underneath half slabs.
Once you place a full slab block over the monster, the monster will stop taking damage.
This video partially demonstrates the issue, although it doesn't include endermen in it.
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However, this issue has been closed as Won't Fix.
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