Repro:
1. Click "New message"
2. Fill in any recipient in the "To" box
3. Enter some subject like "Test"
4. Immediately click "Send"
Result: the email subject will be blank

The bug is that the subject is only populated in the outgoing message if between steps (3) and (4) above you change the active element focus - e.g. after typing in the subject, you press TAB or click the email body text box and only then click "Send".