In a non-spectator mode, I used a timed potion on myself (thrown). I then switched to spectator mode and cleared all effects with -

/effects Bmalchert clear

pressed f5, noticed my spectator head was still emitting particles (though not as much particles as when in another gamemode), and screamed.

Changed to another non-spectator gamemode and particles dissipated shortly after.

EDIT:
When this happens other players cannot to see the potion particles of the spectator, only the spectator can.