Tnt has trouble launching at specific cords such as +255 ~ ~ and -256 ~ ~ or ~ ~ +255 and ~ ~ -256 etc. also the Y cord doesn't effect the launchers reliability 

the tnt will only launch about half to one block away from the launcher but its not completely reliable so it might take a couple attempts to get glitch to work
when creating the launcher it needs to be looking at the next chunk over, as you can see in the photos

this glitch also works in the nether and the end at the same cords as the over world

I'm using a addon to see the chunks but the glitch works without any addons,