Before pouncing gleefully on this question, claiming it has already been answered, no! It was asked, but not answered here:
I have this problem, too, two years later after the last time it was asked. I run High Sierra on a 2010 MacBook Pro. I turn on Time Machine's automatic backups. It says it will backup when the disk is connected. If I direct it to backup now, it searches for my Time Capsule on my wifi network. It finds it, and it mounts it. Then it takes the backup.
On at least one occasion, it did go ahead and subsequently take multiple scheduled hourly backups automatically. However, later I noticed that Time Machine resumed displaying the message that its next backup would take place only upon connection of the disk.
The disk is still mounted! I see it on my desktop and I can browse it. What gives?
The last time someone posted this question, the Community closed it due to spam answers or something. Does anyone have an idea to share now?