Doing /summon PrimedTnt ~ ~ ~ {Fuse:128} will have the PrimedTnt blow up instantly. Probably because it is a bit tag, I don't know.
Linked issues
Comments 3
Working by Design, see http://minecraft.gamepedia.com/Chunk_format#Dynamic_Tiles:
Creeper's fuse is Short, Minecart with TNT's fuse is Int, but TNT's fuse is a byte.
That is probably intended because a Byte has the size of 256 (-128 to 127). A Bit is 0 or 1.
But this is maybe still bugged because it isn't using the negative values for the
Fuse
(see for example MC-77409).