The bug

If you use the /fill command to fill a space with void_air blocks, a 3D grid of the original blocks that occupied that space is left behind. These blocks are ghost blocks, which means that it is impossible to mine the blocks, but exiting and re-entering the world, or going far away and returning, causes the ghost blocks to disappear.

To reproduce

1. /fill ~-16 ~10 ~-16 ~16 ~35 ~16 red_concrete

2. /fill ~-16 ~10 ~-16 ~16 ~35 ~16 void_air

3. Look up and see that a grid of red concrete blocks exists, even though the second command should have removed all of the red concrete blocks.
4. Try to break one of the red concrete blocks. It immediately re-appears after you punch it.
5. Exit the world and then re-open the world. The red concrete blocks are gone.

Expected behavior

Using the /fill command with void_air replaces every block in that region with void_air. (Immediately after this, the void air is probably converted to regular air.)

Observed behavior

Some blocks in the region become ghost blocks instead of void_air blocks.

Video

grid_of_ghost_blocks.mp4