Appointments only show up below for booking or already booked when there is one hour window left.
Say appointment is shown available or booked at 7pm, it will be visible until 5:59 pm for clients as red or blue depending upon status. After 6 pm it only appears in the queue if it is booked and disappears from the system if there was no booking.
For the same reason, appointment availability expires if it is not booked before the last remaining hour. So if you want to book 7pm appointment, you have the option of doing so until 5:59 pm of the same day.