Cannot place blocks outside of the world

Every time I try and use a fill command over a distance of more than 40x40 blocks (on the x and z axes, y doesnt seem affected) I get a "Cannot place blocks outside of the world" error message. This makes creative mode essentially unplayable. I'm aware there has always been a limit (for performance reasons, including both distance and number of blocks filled) but up until recently it was never such a ridiculously SHORT distance. The limit used to be, basically, whatever your render distance was set to. Mine is set to 16 chunks so I should easily be able to fill more than 40x40 blocks. Used to be able to fill something like 100x3x100 at a time.