Here is the 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);
                    }
                }
            }
        }