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There could be several reasons why a promise is returning "undefined". Some possible reasons include:

1. The Promise is not returning anything: A Promise may be defined but if it is not returning any value, it will result in "undefined". Make sure your Promise is returning a value.

2. Improper handling of Promise: When invoking a Promise, ensure that Promise is resolved or rejected. If the promise is not resolved or rejected properly, it will lead to "undefined" return.

3. The Promise function does not have a return statement: When declaring a Promise function, ensure that it has a return statement.

4. Asynchronous Code: When using promises with async code such as API request, make sure you're waiting for the response before returning the result.

5. Incorrect data type: Ensure the expected type of data is console logged using the right method.

To better understand why the Promise is returning "undefined," it is important to analyze your code and find where it is causing the error.