Workaround:

private static void WorkaroundFieldsIssue(RadFlowDocument flowdocument)
{
    foreach (FieldCharacter fieldCharacter in flowdocument.EnumerateChildrenOfType<FieldCharacter>().ToList())
    {
        // only for start
        if (fieldCharacter.FieldCharacterType == FieldCharacterType.Start)
        {
            if (fieldCharacter.FieldInfo.Separator != null && fieldCharacter.FieldInfo.Separator.Parent == null)
            {
                Paragraph parent = fieldCharacter.FieldInfo.End.Paragraph;
                int index = parent.Inlines.IndexOf(fieldCharacter.FieldInfo.End);

                fieldCharacter.FieldInfo.End.Paragraph.Inlines.Insert(index, fieldCharacter.FieldInfo.Separator);
            }
        }
    }
}

ParagraphMarkerProperties of a Paragraph element are not cloned and this leads to incorrect list numbering (bullets) rendering after export since the numbering styles (bullets, numbers and etc.) copy the style properties from there.
This affects Clone() method of the Paragraph and ergo RadFlowDocumentEditor.InsertDocument() method and DocumentElementImporter.Import() methods since they're using Clone() method internally.