Yeah i've tried pretty much all i can think of. She just disappear between 6pm and 8pm just to reappear at 9pm. It's the last event with her, when she invite you on the rooftop to ask if you want to marry her (i usually refuse). Last playthrought i didn't encounter this problem.Have you tried interacting with her on the Street? I'm not sure about that specific event, but a lot of her events trigger when she is on the street. I don't know if this is the same event you are talking about or not, but the event on the rooftop where she shows you the sunrise is triggered from the Street.
Now i messing with the save editor to find a workaround but i'm a bit blind on what i'm looking for ^^