(Experimental mode) /fill command only works from y-level 0 and up in superflat

While working on an aircraft carrier in a Bedrock edition creative superflat world, I wanted to use the fill command to aid in creating large areas. However, since I was building very close to the superflat ground, the y-level was below zero. For whatever reason, the command still recognizes negative y-levels as outside the world, so the command gave the error message "cannot place blocks outside the world". I have not tried this in other circumstances, like in a non-experimental mode, so please let me know if it is just a me problem. BTW sorry for a wall of words