Yup, that’s why I feel it should either be removed or suggested that she knew something was up with her lineage but didn’t know exactly what.
I’d stick to the latter because otherwise the line, “Rey, never be afraid of who you are” doesn’t make much sense, because as far as she is aware Rey is nobody.
Or, you could throw the lineage junk out the window, and just have Leia know she is dealing with that inner darkness. Everybody’s got it. You don’t have to be the descendant of a bad guy to have the potential for evil.
Heck, maybe Leia could have also saw that vision of her on the throne. It doesn’t have to be because she’s a Palpatine.
Difference is that Rey suffers from an irrational, toxic core belief of self-worthlessness. She believes this lie that only others determine as to whether she is valuable or not, and due to her lack of self-value she relies on others’ validation to feel happy and push away her feelings of self-worthlessness. Her relation to Palps is there to reinforce this, the moment she stabs Kylo is when she becomes convinced that she is meant to be consumed by the dark side in the way her grandfather was, that her heritage is the reason she is falling to the dark side and once she does turn then the Resistance is no longer going to give her any validation.
Rey being Palps’ granddaughter and her exiling herself because of this is sort of like other movies in which the end of Act II is the low point for our protagonist(s).