I've either found a bug (which has been around since at least San Diego) or expected behaviour (which it must be if it's been around since at least San Diego ) with the REST step in a Flow Designer action.
Essentially, when adding query parameters (or headers) you can use fixed values, data from other steps or script. However, if you add a scripted parameter you can no longer add values from other steps (using the data pill picker or dragging from the data column). I've uploaded a video showing the issue to Youtube (
https://youtu.be/Q9rScM1z8Og)
I've tested this on Utah, Tokyo and San Diego and the issue is repeatable. Also tried on Firefox as last year there was an issue with Chromium based browsers when using data pills in the resource path of a REST step.
I'm not expecting a solution, but if anyone has come across the issue or read something then I'd be interested. The workaround at the moment is to build all the parameters needing data pills before adding the scripted ones disable the scripted parameters and then re-enable.
