Blocks at y=-64 don't render on a map

The bug

Blocks at y=-64 don't show on a map (item), but their shadows (and highlights) still do.

Work around

Place transparent blocks (barrier blocks, glass etc) above the affected area. (Warning: This can cause a huge amount of lag if done all at once (another bug?))
This however has the side affect of removing the void background colour (gray) (another bug?) and removing their shadows and highlights

Work around shown by Alex Eade
Comment: https://bugs.mojang.com/browse/MC-134002?focusedCommentId=933604&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-933604