I was planning to update the TestFlight version of Scriptable to the App Store version on my iPad (but keep the TestFlight version with HealthKit support on my iPhone for now) to “buy” the App (and maybe then return to the TestFlight version if Simon allows us to keep beta testing updates).
However, after tapping the Update button a small circular progress bar flashes only briefly after which the Update button returns…
You can update on your iPhone and then switch back to the beta - that’s what I did 
Somehow I can’t…
Weird new thing in iOS 12: previously I had an “Install” button in the App Store for Apps I was testing via TestFlight. Now that button is called “Open”… (and opens the App instead of installing the App Store version)
Mine did the same thing. I had to delete the beta version to install the app store version. Scripts are all cloud synced so they shouldn’t disappear.
I had the same thing. I had to uninstall the scriptable beta and re-install from the app store.