Redstone lamps do not show on maps

The Bug

Redstone lamps do not show up on maps. Instead, the block below them is displayed.

Steps To Reproduce

1. Place a large amount of redstone lamps:

   /fill ~1 ~ ~1 ~9 ~ ~9 minecraft:redstone_lamp

2. Create a map.
3. Look at the filled map and take note of whether or not the redstone lamps are on the map.

Observed Behavior

Redstone lamps are not shown on maps.

Expected Behavior

Redstone lamps would be shown on maps.

Code Analysis

Code analysis by [Mod] Avoma in this comment.