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There are a few possible reasons why a Firebase function triggered by a database event might return undefined instead of a Promise or value:

1. The function might not be returning anything explicitly. In this case, the function will default to returning undefined. To resolve this, the function should return a Promise or a value.

2. The function might be returning a Promise, but it is not resolving correctly. This can happen if there is an error in the function, or if the Promise is not properly constructed. To resolve this, ensure that the Promise is properly constructed and that any errors are being handled correctly.

3. The function might be asynchronous, but it is not properly handling the asynchronous nature of database events. This can happen if the function is not properly awaiting the database event before executing. To resolve this, ensure that the function is properly awaiting the database event before executing.

To further diagnose and resolve this issue, it would be helpful to review the code and any error messages or logs that are being generated.