Our copy of Mansion of Madness, second edition doesn't contain the original 8 investigators cards, is this correct?
It seems to be correct. From the second-edition rulebook, page 4, "First Game" sidebar:


  Before choosing a scenario, if you own the first edition Mansions of Madness base game and/or any of its expansions, you can integrate the components of those products as described by the Conversion Kit rules insert.
  
  Players can play any investigators from the integrated first edition products, and the app can use any of the monsters or map tiles from those products during the game.


I think it's reasonable to infer that the original investigators (and monsters and map tiles) are intentionally separate from the second edition.

I don't own the game, so I can't physically confirm this.