This is common in estrangement. I doubt they forget i think its more dont care.
Often the SO and even family are a big part of it. We adopted a boy frim Asia when he was six and he estranged all of us after he got married.But it started after he met her. I have NO doubt that his wife was part of it. He worshipped her and she wanted him to herself and he was not strongly attached to us (or anyone) because of his six years in an orphanage .So he left, didnt explain why, and that was that. He isnt with his family much, works 80 hours a week. He has two boys I never met. But is this on his wife??
To me, he is now 40 years old. He was 28 when he married, an adult. He is brilliant, has his own company, at least a mother in law, and never even went to college. He made the decision to go along with her. And anything written to him by me was unanswered so I got therapy and was able to learn to take care of myself, lean into my four other loving kids and my husband and build my life without him.
In the case of your daughter the SO certainly probably talked trash about you but she chose to agree with him. I dont think you can fight them all. I would get therapy and decide where to go from there. Maybe lean on your kind loved ones, friends, etc. And learn to be good to yourself. For now there is nothing you can do to change them and grandparents have no legal rights in the U.S. for the most part. They did once but the Supreme Court took them away unless your own child dies or if the grands lived with you for an extended period of time. I checked. All states are a little different but not that much. For now you need to be able to let go with love and find happiness. You have no control over your daughter or anyone but yourself. Why not choose to be with those who are kind to you? I realize how hard this is but you dont really have a choice. I am sorry.
Love and light!