The Grandfather Paradox makes the plot fall apart
Seo-Yeon in 2019 speaks to Young-Sook from 1999 and asks her to save the dad. Young-Sook rescues Seo-Yeon's dad from his death in 1999. That means Seo-Yeon from 1999 grows up happily with both her parents, and in 2019, she does not tell Young-Sook to save her dad.
In any of the scenarios where Seo-Yeon is tortured/hurt/attacked by Young-Sook in 1999, if she survives and grows up to 2019, she simply won't have the same conversation with Young-Sook.
This is not a problem with movies like T2, where the time-loop is treated as predestination. John Connor sends Kyle Reese back in time, who impregnates Sarah Connor, who gives birth to John, who sends Kyle back in time, who impregnates Sarah...
Even in the Back To The Future movies, you can always explain away this loophole by assuming Doc and Marty are transported by the Delorean into an alternate timeline. But even that explanation doesn't make sense here because little Seo-Yeon's life course is also majorly altered by the time travel, but how does she still end up in the same house at the same date EVERY SINGLE TIME? Especially in the timeline where both her parents are killed, meaning they never even bought the house, so how does Seo-Yeon still end up in a house that she never even lived in?