When sending emails, the copy stored in the "Sent" folder is marked as "unread" (thus making the "Sent" folder marked in the folder list). It is needed to mark them as "unread" manually. This is using Kmail 5.8.80 beta1 on openSUSE Leap 15 with the devel KDE repos activated. Previous to this version, the behaviour was to store the sent emails marked as "read, sent". Incidentally, note that the "sent" mark has also disappeared in the beta version.
Thanks for the report. This should be fixed in the final KDE Applications 18.08 release (Kmail/Akonadi 5.9). Please reopen this if you still see the problem on stable release.
I can confirm it's fixed (though the "sent" flag is still missing as far as I can see?).
Never mind, "Sent" seems to be set in newly sent messages. Must have been looking at old messages. Sorry for the noise!
With kmail 5.12.2, *some* sent mails end up unread in the sent folder, akonadi postgres backend, with cyrus imap server (openSUSE 15.0). There's no clear pattern visible, but just opening the mail doesn't help. Those mails need to be flagged "read" manually, and the state icon is different. See attachment. SOFTWARE/OS VERSIONS Linux/KDE Plasma: openSUSE Tumbleweed 20191030 KDE Plasma Version: 5.17.1 KDE Frameworks Version: 5.63.0 Qt Version: 5.13.1
Created attachment 123674 [details] middle icon show such an "unread" mail, after manually flagging "read"