Monsters Affected By Environment/Weather Under Slabs

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.