I had built a message extension on Microsoft Teams which opens an Adaptive card.
The placeholder in the dropdown is not visible in case of Android app, but visible for iOS and desktop app.
Please refer the screenshot below:
As per the schema, Input.ChoiceSet don't have the property placeholder. However, the issue is tracked by the SDK team on the platform support discrepancy. The desktop client and iOS client have currently hardcoded the default value to be "Select an option" when there is no selection in choice selection. This is by design.