Carpets do not always appear as expected on maps.
When placed on top of a solid block, the carpet appears on the map.
When placed on top of a block that does not render on maps, the carpet will not appear on the map either.

Screenshot shows a map and a few tests with carpets.

Steps to recreate:
1. Create a superflat world of glass
2. Use an empty map
3. Place carpets, and they will not show up on the map like they would on a grass superflat world

_{Failed to find duplicate bug with words "carpet, invisible, map"}