I just got word back from GitHub. They don’t automatically run Jekyll builds when your scheduled posts are set. This means if you have
future: false enabled, your future posts won’t be published.
Which means there’s a gap I can fill. I’m going to try making a Probot app, but first, or an app on the GitHub marketplace to enable this. This is a need I know a lot of people have; let’s see if I can manage to fix it.