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I have tested on my side and noticed the same behavior you are seeing. This behavior is expected with NBS Search because it does not provide a standalone Search Box web part. In your case, you are using a Search Refiner Web Part, not a Search Box, which means it does not function as an independent input field.
In NBS, the search input is part of the overall Results/Refiner component and only renders when a valid query is present. If the “Search query keywords” field is left empty, the web part treats it as an uninitialized or not configured state, so the entire component, including the search bar and its placeholder does not appear.
Adding a space or any value works because it creates a non-empty query, which allows the search pipeline to initialize and the UI to render. As a result, there is no option in NBS to display the search box with placeholder text without providing at least a minimal query value.
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