VB.NET App.config values resolving differently in production: machine.config inheritance or stale My.Settings defaults?

In a .NET Framework Visual Basic project, App.config is copied to [AssemblyName].exe.config at build time, and runtime reads go through ConfigurationManager (appSettings, connectionStrings) or the My.Settings proxy generated by the Project Designer. A value that resolves one way on a developer machine can resolve differently on a production server, and several overlapping configuration layers could account for the gap.

• machine.config inheritance: machine-level settings can conflict with application-level keys, with behavior that varies by .NET Framework version.
• My.Settings defaults: strongly typed settings carry compiled-in default values, so a stale or missing deployed exe.config may not match local behavior.
• CI/CD transformation gaps: config or settings files that are not transformed during the pipeline leave production reading developer values.
• Runtime mixing: a project that also targets .NET (Core) replaces App.config with appsettings.json, and mixing the two systems can raise runtime exceptions.

The exact precedence between a duplicate appSettings key in machine.config and the application's exe.config should be verified against current framework documentation before relying on either value.

Which layer should be diagnosed first when a VB.NET appSetting resolves differently on the server? When machine.config and the deployed exe.config define the same key, which value does ConfigurationManager.AppSettings return under .NET Framework 4.8? And does My.Settings fall back to its compiled default when the deployed config file is absent or outdated?