Is it about the pendant’s existence?
I also confused at first.
But after re-read, I got a conclusion:
In Lucia’s dream (seemed to be given by the magical power of the pendant. How it’s activated would be explained later), Lucia fell from the tree & one of her friend (Rossa) also died. Lucia’s mom was forced to sell the pendant to pay for Lucia’s medical bill. So, when Lucia lived her life in the dream, the pendant was in the pawn shop. Until Lucia found it later in an auction & then gave it to her uncle.
In reality (the life Lucia lived until now after woke up from the dream), Lucia didn’t experience the falling from the tree. Thus her friend Rossa was still alive when Lucia’s mom died. And when Lucia was brought to live in the palace as a princess, she still got the pendant with her.
But somehow it seemed, after Lucia dreamt a long dream about an alternate life (when this dream happened, Lucia had already lived inside the palace), the pendant suddenly disappear without Lucia even notice.
Maybe because she’s overwhelmed by the long dream, Lucia didn’t realize about the disappearance of the pendant, plus her memories were mixed (about whether she fell from the tree or not, and whether her mom sold the pendant or not, etc.)