Well, I agree missing an appointment is unusual. It's hardly criminal, though.
My ex and I have very open custody and often he has our daughter on days she is to see the orthodontist. He has TWICE missed her appointments even with my reminders (or nagging, depending on how you see it, lol). Doesn't make him a bad dad, just a space case. And Ayla's dad was pretty new to being a parent, from what I have gathered.
And as I have stated before, I used to check on my daughter before I went to bed, which was usually 2-3 hours after she was put to bed and I did not get up during the night to check on her. That would mean she easily slept alone, in her own room, in her own bed, for 10 or so hours without being checked. (This was when she was around Ayla's age)
And I am a darned good parent.

Every parent and every child is different. A good parent meets their child's needs. It is honestly that simple.
I am not saying dad isn't responsible for Ayla's disappearance, and I believe he should be considered a POI, but the child sleeping for 12 hours isn't proof of anything, IMHO.