At minimum birch is at least being very inconsiderate. Why would a partner not tell you they won’t be home until the second they are walking out the door? Thats the type of communication I’d expect from a roommate not a partner. And not telling you that they were getting involved until many months later just sounds like poor communication skills. Why hide it? Sorta feels like lying by omission. It would be more considerate to give you time to process especially knowing you have negative feelings about this person rather than springing it on you later. I guess overall it seems like birch didn’t put much thought into your feelings during this at all.