First things first, I'm working with the Common Data Service with the Dynamics 365 Sales System on the backend. Sometimes people reply to questions like "Sharepoint doesn't support that", and I will not mark those as a valid answer as it assumes I am using a SharePoint back-end. A lot of things seem more straightforward with SharePoint as your data source.
I'm noticing a difference between my PowerApp when I am testing it within make.powerapps.com, and how it behaves when called out in the wild. Given I have an application that creates a collection of records, and displays them in a Gallery. It's a custom gallery, so some pieces of information are displayed as labels while others are displayed as input fields. Keep in mind, both situations have the same "Run On Start" so it is not a data difference. The labels just have the Text as "ThisItem.FieldName.Text" and are read-only. Whereas the Inputs (like a dropdown) have the default as ThisItem.FieldName.Text and the Items formula defines the choices (usually from an Option Set or a list of records from another entity in the case of a Lookup field.).
From PowerApps, pressing play to test, all the items in my gallery have data, everything works as designed.
When called directly (URL) or by opening the PowerApp to use it, the field labels all have values, but the input fields do not seem to work. When I say they don't work, neither the Default Value nor the Items choices are available on any row except for the FIRST one in the gallery.

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