There are no known issues with merging hidden fields to PDFs, we merge hidden fields daily in our own forms…
If you are using conditional logic to hide the Dropdown field, that would cause the hidden field to be blank… other than that, the hidden field data should merge.
Use this form if you are able to provide a link to the form, or create a PRO2PDF template of the merge, or access to the installation so that we may further investigate.
We remain at your service.