It's mentioned in the film that he was basically in rehab for his alcoholism at that point, and he approaches Katniss after he dries out. This is why he says he disagrees with President Coin's draconian view towards alcohol: he can't get booze in District 13.
This is the first time that Haymitch is mentioned in Mockingjay (the book):
I haven't seen Haymitch since I attacked him on the hovercraft, leaving long claw marks down his face. I
know it's been bad for him here. District 13 strictly forbids any production or consumption of intoxicating
beverages, and even the rubbing alcohol in the hospital is kept under lock and key. Finally, Haymitch is being
forced into sobriety, with no secret stashes or home-brewed concoctions to ease his transition. They've got him
in seclusion until he's dried out, as he's not deemed fit for public display. It must be excruciating, but I lost all my
sympathy for Haymitch when I realized how he had deceived us.
-The Hunger Games: Book Three - The Mockingjay, Part One: "The Ashes", Chapter Two.
Then he appears when the filming is being done:
"Action!" So I hold my bow over my head and yell with all the anger I can muster, "People of Panem, we fight, we dare, we end our hunger for justice!"
There's dead silence on the set. It goes on. And on.
Finally, the intercom crackles and Haymitch's acerbic laugh fills the studio. He contains himself just long enough to say, "And that, my friends, is how a revolution dies."
-The Hunger Games: Book Three - The Mockingjay, Part One: "The Ashes", Chapter Six.