I have never seen this mentioned anywhere so I just assume i'm thinking about to much, when as a force ghost of Rishi, he says 'Dark times are coming and you may forget why you are fighting' Expansions aren't though up on the spot, and usually happen years in advance.
My theory is he saw the Emperor return and knew about when would happen, and he knew we would have to choose between the Empire or Republic, I don't know, its just some headcanon I have.
And to why he didn't warn us? Name one Jedi force ghost that was straight forward.