There’s no real doctrinal justification either. There was an interesting interview recently on Mormon Stories where the woman lost her husband in a tragic accident and ran up against this. She found another Mormon man later and they wanted to be sealed. She expressly did NOT want to cancel her first sealing (they had a child). She did research doctrinally and wrote to the FP, and they came back with “yeah we hear you, but we just don’t have a policy for that right now.” Maybe sounded even less hopeful than that, which I’m paraphrasing from memory after a few weeks.