Transparent blocks at y > -64 causes map to show said blocks' color instead of the void color

The bug

When creating a map of the void the map is gray. However, when a transparent block (e.g. barrier or glass) is placed at y>-64 above the void, the map becomes transparent instead of showing the void color.

This is closely related to MC-113292 and MC-165778, possibly one being the root cause of the other ones.

How to reproduce

1. Create an empty superflat world.

2. /fill ~ 0 ~ ~20 1 ~20 barrier
   

3. Create a map.
   The area with the barriers is shown as transparent.