Non-solid blocks don't show up on maps if the block below isn't solid

Not sure if non-solid is the right description.

To reproduce, place a bed on an elevated platform and dig away the platform. Then create a map and observe the bed not show up on the map.

Alternatively, place a weighted pressure plate (iron or gold) on a fence and create a map of the area. Observe the weighted pressure plate not show up on the map.

Weighted pressure plates and beds usually show up as their own color on maps.

Likely also affects other non-solid blocks that usually show up on maps.