Not able to save fillable PDF without losing data in expandable fields

US FDA provides a fillable PDF form called USFDA MedWatch form 3500A. There are few fields in the form which allow to enter unlimited text. There is a plus sign which when clicked opens up a scroll bar for that field which allows to scroll through the text. My issue is that I am not able to make this form a non-editable one after completion. With any method of saving, full text is not visible in those fields where there is a + sign. I got an error message as follows 'the document has been saved, however, it could not be converted according to the selected standard profile: Convert to PDF/X. Please use Preflight witht he profile "Convert to PDF/X" in order to identify those properties of the document which prevent it from being compliant to this profile'. Attached is the template.