I wouldn't say this dog is angry or resentful or anything of the sort. It's pretty unlikely she's thinking "hey you left so now I'm not going to listen to you any more and there's nothing you can do about it". She's a puppy and the OP hasn't been training or interacting with her for months, someone else has been. The first step is the OP just needs to reestablish a bond with the dog. Ace and the roommate's puppy should be separated more than they are together, since raising two puppies together can discourage both from having a good relationship with the humans in the house.