I was working on a redstone contraption involving maps in a void world when i noticed that placed blocks at y=-64 showed transparent (and produced the same color gray as the void underneath). I was in a lightly modded version (forge, worldedit, and a datapack extending the build limit to y=1024) so i tested it in a pure vanilla install and it still happened.

steps to reproduce
1: open void world
2: place blocks at the lowest elevation possible, y=-64
3: place different blocks next to them at y=-63
4: get a map and open it

observed results:
the blocks at y=-64 dont show up. in the included image i used green and red blocks. the green blocks become the same color gray as the void, however it still applies the shading effect (the 1 pixel wide dark gray line). the blocks at y=-63 show as expected

expected results:
the blocks at y=-64 should show normally as they would on any other y level. (in the demonstation image they should be green)

image 1: demonstration setup with the green blocks at y=-64 and red at y=-63

image 2: the result on a map. notice no green pixels, but there is still a line from the shading effect