Alright, I think I have my theory for what's happening here:
Yvonne experienced a world where none of this happened, the first time around. She grew up on her own, and got to school, met Elsa etc, etc. And eventually, through all that, and eventually got sent back in time to this arc we're having right here.
But Elsa, in the timeline we already experienced,
REMEMBERS ALL OF THIS. Somehow, in Elsa's universe, she experienced a world where Yvonne saved her and made the promise. But I don't think this is just yvonne- I think this is
Everybody in this World.
Recall the earlier chapter, where Elsa said Michel helped arrange for her parent's to have a lighter sentence, and Yvonne goes "Huh, when did they get so close?"
We're seeing that now too. Yvonne seems to be the only person in the original timeline who does not have memories of this time period she is now rewriting.
So basically, Yvonne experienced a timeline, but then later overwrote it, so she was the
only person who recalls the timeline before it was overwritten, even before she overwrote it. Later on, she's going to get sent back into the present, possibly even at the point where she saves her from falling off the building.
Diagram to explain: (O) Original timeline, (X) Overwritten Timeline, (Q) Shifted Timeline
Original Timeline: OOOOOOO(Yvonne gets sent back)XXXXXXXXXXXX(Yvonne returns to present)QQQQQQQQQQQ
Yvonne's Experience: OOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOQQQQQQQQQQQ
Does that make sense?