/fill command filling exactly half of the specified space

I have experienced this multiple times now. I try to fill a space with the /fill command, but only half of it fills. I try the command again, still says 0 blocks filled. I checked if I made some mistakes, but I didn't. I provided some screenshots if that helps.

The layer of birch planks shows the area i wanted to fill with glass blocks, and you can see that exactly half of it fills up with glass.