When you try to place blocks below y = 0 or above y = 255 with /fill, then you get an error message saying Cannot place blocks outside world.

I got it to work, with a bit of a delay, for Y=257, but it gets the error message at Y=2048. Commands used were
/fill 300 257 150 350 257 200 stone
and
/fill -50 2048 -50 50 2048 50 dirt
Worse, the 257 behavior cannot be replicated, and now I am getting the error message at BELOW Y=256 and as low as Y=96. It might have something to do with the number of blocks to place, or it could be whether there had already been an error displayed.

Turns out vanilla has some really strange logic there:

it checks if the block at Y=(maxFillY - minFillY) is loaded.

It's probably the wrong constant being replaced somewhere.