It is not RNG.Thanks for the note about the Event Logger. Because of the first 4 screens in the ? I had assumed that this was just another reminder-help text screen.
It does show The Cave as an event. Although several days have passed (a week?) and no follow-up despite talking to Luna at all times of day. If this is just an RNG it could just be a low-probability long wait time.
1 week and then all you have to do is go to your room at noon or afternoon.
As @Enyos said, it's because she has to learn to use the high heel and muster the courage to ask for it.
It would be weird if the quest triggered a day after she went with isabel to learn to walk with the shoes