She ate last night because Gu Qingcheng told her what time dinner would be served.
Gu Qingcheng had been busy all morning and didn't have time to care about Yun Liuli, so Yun Liuli obediently stayed in her room and didn't come out for breakfast.
According to the rules Gu Qingcheng set at home, when he was busy with work, the housekeeper wouldn't disturb him, even at mealtime. The food would be kept heated until Gu Qingcheng went downstairs and said he wanted to take his meal, then the food would be brought to him.
The housekeeper was already used to such an arrangement…
Today, Yun Liuli didn't go downstairs all morning. The housekeeper must have thought that Yun Liuli, as a kid, wanted to sleep in a little on the weekends and thus didn't call her, wanting to let the child sleep a little more.
Gu Qingcheng ate his breakfast early, so Yun Liuli didn't get to eat with him.