Chunk sections that only contain air and cave air are treated like sections that only contain air.
However, air and cave_air are different blocks. The difference of these blocks can be observed for example in fill commands or with parrot mob spawner blocks (cf MC-232359).
The bug causes parrot mob spawner blocks to see air instead of cave air when the chunk section is empty. The bug only seems to be observable with fill commands in empty chunk sections, e.g. the command
/fill ~ ~ ~ ~ ~ ~ stone replace cave_air
fails when the section is empty, even though there is cave air at the current position.