Could you post a screenshot of what the user sees when trying to view the data?
Is the Guest user assigned a site role that would preclude their ability to interact with the underlying data?
Finally, what are the permissions settings for the project and workbook in question? i.e. are Guests/All Users allowed to view or interact with the Data Source?
I actually "fixed" the original issue. My brain was not functioning fully. I revised the original post with a different but related question.
Thanks for updating the OP
It's hard to say what exactly might be happening from the description and screenshot you've provided. My instincts are to check site roles and permissions for the project or workbook.
What fields are missing for the web app user? Could you post some screenshots to contrast what the web app user is seeing vs. what they should be seeing?