Unfortunately this is not possible (yet). It's a known limitation of Shortcuts in its current 2.0 implementation.
Federico Viticci mentioned this in his (epic) iOS 12 review as something he'd like to see in the future:
https://www.macstories.net/stories/ios-12-the-macstories-review/12/#content
I found out that my shortcuts were working on iOS 12.0.x and after updating to iOS 12.1 they stopped working. I had to work on the flowcharts to get them working again.
