odd display (of not) of message subject

I consider this a curiosity, not a real problem.  In the (threaded)  
message list, it shows a subject of "reports" which I do see as the  
"Subject: " when looking at the headers in the source of the message.   
However, in the main message display header area, the Subject: says  
"(No subject)" even if I show all headers.  The last few lines of  
headers are:

Mime-Version: 1.0 (Mac OS X Mail 11.5 \(3445.9.1\))
Subject: reports
Message-Id: <[email protected]>
Date: Sun, 3 Mar 2019 14:00:31 -0500
To: Jack Ostroff <[email protected]>
X-Mailer: Apple Mail (2.3445.9.1)
Status: RO
X-Status:
Content-Type: text/plain; charset=utf-8
Content-Transfer-Encoding: quoted-printable

This is a single part message, text only, no html or attachments.  I've  
got plenty of other messages from the same sender which properly  
display the subject.

Any idea what's going on?

Jack