Isn't it more like she can't recall hem anymore? Obv, what she remembers from them stays as long as she still remembers the memory itself but everything else is *puff* gone. Isn't that why when she touched Doyun's hand again she couldn't recall her 1st life?
It's not weird but it's her stability. She kept remembering details in every birth cause she separated with each family in each birth and she had to strive hard to prepare and survive in the lives ahead with new world, new ways of means. In 18th life things changed for her. For the first time she got a reason to look forward to her next life. It wasn't survival this time. It's the same as when we are given a comfortable atmosphere and familiar faces, we tend to let go of our past burden making it easier to survive.
I found so weird that she was like yeah I forgot my past memories puf. To what degree ? Maybe I’m missing something because I hacen read this in a while. But she talks like she actually remembers.