If you go to a world and then type a fill command to a specific area, and then there are 3 options after the block id and type,[hollow,destroy,keep,outline]

The outline sub-command doesnt work instead it does the same as hollow.

To reproduce,
1. Go on a world and have cheats on
2. Copy paste the following command
3. '/fill ~10 ~ ~10 ~-10 ~10 ~-10 planks 1 outline
4. observe
5. now copy this other command
6. '/fill ~10 ~ ~10 ~-10 ~10 ~-10 planks 1 hollow
7. Move some blocks away from the generated structure
8. Enter the new copied command
9. Observe, the end result will be same

Provided are screenshots to show what is supposed to happen.

Click to view better