After implementing dark and tinted icon variants as per Apple’s documentation, devices running iOS versions prior to iOS 18.0 display the Dark Mode icon instead of the expected default light mode icon. As well as TestFlight showing the Dark mode icon.
Human error.
The DevOps build configuration in the pipeline was targeting macOS 13…, looks like the build may have ignored the "Any Appearance" and "Tinted" asset types, as they were either not recognised or not properly handled for that platform. As a result, the Dark Mode image was used as a fallback.