Using "/summon" creating a PrimedTnt setting "Fuse"Not less than 128 makes it explode immediately.
Run

/summon PrimedTnt ~ ~ ~ {Fuse:127}

An activted TNT will spawn and explode after 127 ticks(6.35s) as expected.
Run

/summon PrimedTnt ~ ~ ~ {Fuse:128}

An activated TNT will immediately soawn and explode without delaying.

It seems to be DataType "Fuse" is BYTE(1 Byte) instead of SHORT(2 Bytes).