Please excuse the clunky Subject header, I couldn't think of a better one.
In the "New Outlook" start a draft email, go File --> Save As and save it anywhere, for example the desktop, and then open the .eml file. You will see your contents saved, great. Now go to a PDF, select a line and copy that line and then paste it into your draft. Add a new line of text under that copy and paste line and then go File --> Save As and open the .eml file. You will see the new additions are not saved. Even if you do a paste without formatting the issue will still happen. No matter how many additional lines you add when going File --> Save As it will not save the file with the new lines. The only way to get it to save with the new lines is to close Outlook, open Outlook and then open the draft again to then save it.
I am unable to reproduce this issue in the current Outlook client and it feels like some weird file lock issue.
Any ideas? Anyone else seen this? I have reported it as a problem using the Feedback option.