Validation Error when using a dynamically generated PDF

Hello, we have a scenario where we use ABCpdf to load a PDF document, populate form field values in the document and then use the document in an eSign package. When we try to send the package we receive a "Bad Request" response back:

Could not upload document to package. Exception: The remote server returned an error: (400) Bad Request.

Optional details: {"entity":null,"packageId":null,"messageKey":"error.validation.verifyDocument.hasNeedAppearancesFlag","technical":null,"message":"This document has the \"NeedAppearances\" flag set. This has negative implications concerning the security of the signatures, and cannot be accepted.","code":400,"name":"Validation Error"}

The error message indicates the NeedAppearances flag is the cause of the exception. But when debugging and inspecting this property in the ABCpdf object I see the NeedAppearances property is set to true before populating the form fields. Using a document without populating form fields, where NeedAppearances is already set to true, the exception does not occur. Next, I tried populating the form fields and then set NeedAppearences to false, but I still receive the same exception. I don't know what conclusion to draw from this. Any insights to what is going on?