The particle command usage is :
/particle <name> <x> <y> <z> <xd> <yd> <zd> <speed> [count] [mode]

But when you try to autocomplete the [mode], it don't work, but it autocomplete when you are at the [count] level.

/particle explode ~ ~ ~ 1 1 1 speed count
Pressing TAB don't do anything

/particle explode ~ ~ ~ 1 1 1 speed
Pressing TAB try to autocomplete with normal/force