[Exim] Handling of subject header

In sending out a series of test messages using -t to get header info from the text file being transmitted. Sometimes the 'Subject:' info appearing at the top of the file is shown as the subject at the receiving end, removed from the text, and sometimes not. The text file itself if a copy of earlier mail itself, with preceding header info appearing later in the text

On the command line, I write 'exim -t <textfile

How does exim handle this? I don't see this topic discussed in the documentation. Any thoughts regarding the seemingly inconsistent treatment?

David