Why are my uploaded files in the vector store being deleted after some time?

I used the below code and successfully uploaded the files to vector stores.
But after a while, the files had been deleted not only from vector stores but also from Storage.

     # Step 7: Upload a file to openai
    # https://platform.openai.com/docs/api-reference/files/create
    message_file = client.files.create(
        file=(file_name, file_stream),  # Pass the file stream directly, 
        purpose="assistants"
    )

    # Step 8: Add the uploaded file to vector store
    vector_store_file = client.beta.vector_stores.files.create(
        vector_store_id=found_vector_store.id,
        file_id=message_file.id
    )

Please advise if you encounter this issue and how to resolve it.