Thanks for your patience. It is to inform you that our product team has completed the work on this issue (WORDSNET-11393) and has come to a conclusion that this issue and the undesired behavior you're observing is actually not a bug in Aspose.Words. So, we have closed this issue as 'Not a Bug'.
A form field is represented as several nodes inside: FieldStart, BookmarkStart, Run, FormField, FieldSeparator, Run, FieldEnd, BookmarkEnd. You should set font properties for all the parts of the field except the bookmarks to get correct result in ordinary and design modes. Please use following code example to achieve your requirements.
Document doc = new Document(MyDir + "FormFields.docx");
Field field = doc.Range.Fields;
Node node = field.Start;
while (node != null)
if (node is Inline)
Inline inline = (Inline)node;
inline.Font.Size = 10;
inline.Font.StrikeThrough = true;
if (node == field.End)
node = node.NextSibling;
doc.Save(MyDir + "Out.docx");