you should read:
https://www.mangago.zone/read-manga/my_dear_aster/
it's about a mom who wants her daughter back...
I 100% understand and agree it always rubbed me the wrong way in stories like these. There's actually a manhwa similar to what you described but it was a little different. If I remember correctly both the ml and fl got sent back in time and fixed there relationship. They had one child and loved them, but when they had their second child it turned out the be the reincarnation/same child from their precious life. And they even said they love their family but it felt a little special having their first child back and it ends with the three of them hugging. It just left a bad taste in my mouth, but I wish I remember the named of the manhwa.
I believe the deal is that Luka's soul will enter the baby's body whenever they gain a soul. So biologically and physically the kid will be different, but the essence will be the same.
Hmmmm while i love the story it always bothers me when in these kind of stories the FL almost always expects her child to be the same one that she had in her past life, i mean that child was born between her and the former husband, so if the father is different wouldn't it make sense that the child would be different as well, well i guess in this case one of the humunculli may be Luca but we'll see about that, but i have to wonder if a random FL gave birth to a child and it's not the same one she had would she be dissapointed that it's not the same child, would she still love that child or would she still be stuck in the past searching for her first child and possibly neglect her current one, i think a manhwa with a plot like that would be kinda interesting, well i can't wait to see how this story will unfold ╮(￣▽￣)╭