Apparently pushing a High Sierra combo updater is not possible, because someone at Apple decided the user needs to interact with the updater.

We went several rounds with Jamf and Apple, tickets here, tickets there, and when the dust settled, the determination is that High Sierra combo upders require user interaction.

This, from Apple enterprise support, confirmed by Jamf.

@dan.kubley of Jamf fame to the rescue as usual, provided a workflow, which is to push the Install macOS High Sierra.app full installer.

Tested. Works. #ourHero

To Apple, since we know you use Jamf Pro...

I feel bad for our Apple Rep, he gets an earful from us when we run into these kinds of bizarre complications...he is he best of the best, but getting an earful from us and other companies.