In my previous test I did not close Thunderbird before reopening the signed and encrypted draft message. this time I did it and nothing changed: The right title of the draft was still there.
I prefer to stick to Stable but my main laptop is fairly new, that is why Buster is installed on it. So on a previous laptop, french Debian Stable with all updates installed, with contrib, non-free; proposed-updates, updates, security-updates, backports enable (but no backport package installed) Same test and same result as with Buster on my new laptop: the title is correctly saved and correctly sent after reopening Thunderbird. In case it could be significant: - I use Gnome desktop environment (if you use a lighter DE or a WM it would be possible that a library that improves Thunderbird/Enigmail (but is neither considered recommand or suggest) is not installed. - I used my gmail address, so it's an IMAP remote storage of the draft. Sorry, I have no other clue...
