Verification builds:
1.2.5.15 Retail

Summary:
Having a layer of transparent blocks (such as glass) will prevent endermen spawning in on valid platforms below, even if the glass layer is much higher than the platform.

Steps to Reproduce:

1. Travel to an area over the void in the end
2. Set difficulty to Peaceful
3. Place a single layer of glass at y100
4. Place a single layer of end stone directly below the glass at y50
5. Set difficulty to Hard
6. Wait for mobs to spawn on the end stone platform

Observed Results:
The glass prevents any spawning on the valid blocks below. Endermen will not spawn on the platform all the while there is glass above.

Expected Results:
Glass should not prevent endermen spawning on valid blocks below.

Screenshots/Videos attached: Yes

Notes:
Tested with other transparent blocks, they also prevent mobs spawning below: Leaves, pistons, fences, but apparently not bottom-half slabs (see MCPE-28626)