how adobe display content tree on the tagged flattened pdf?

You are trying to do something that Acrobat will not do. When you flatten a document, all bets are off regarding of how the informaiton is represented within the PDF document. As long as the visual representation is the same and all other features work as advertised (e.g. the tagged structure of a document), Acrobat can do whatever it wants in regards to how the file is saved. You could potentially draw all capital A glyphs in the document first, followed by all "B", and so on, continuing with the lower case letters, the numbers and other characters. This would still be a valid PDF file. Extracting informaiton from it would be much harder, but it would still be valid. This is a case of "it is what it is", and there is nothing you can do to change Acrobat's flattening behavior. 

To give you some more information about what's going on: When you compare the contents of the "containers" you see in your "before" and "actual" documents, you will very likely see that they are identical. There actually is no form field in these containers, they are stored in the "Annotations" hierarchy. This means that based on the content, there is no relationship between the static text and the form field. What happens when you flatten this document, is that Acrobat adds the flattened form fields after all the other elements on that page - and that is what you see in your "actual" screenshot.