I saw in the PowerApps ideas site that a 'Run button for OnStart' was added to the ribbon. While I have found it, I expected being able to click it in order to run the OnStart function, however that does not seem to be working for me?
No sorry I think you misunderstood. I know how the OnStart function works as well as the OnSelect function for buttons.
Let me give you an example:
OnStart: ClearCollect(Collection, Datasource)
-> After OnStart is completed, app screen displays collection data in a gallery.
If I click 'preview' within PowerApps the OnStart function does not actually run. Therefore, in order to 'preview' properly including the OnStart function I create a button I can click that replicates the OnStart function in preview.
When I saw this 'idea' implemented I expected that the OnStart ribbon action allows me to click it and run the OnStart function - eliminating the need to replicate this using a button.