Sometimes when I delete or move multiple emails from the same folder, kmail copies the mails into the destination, but does NOT remove them from the source folder... but when I look in the source folder with any other imap client (i.e. thunderbird) the mails are actually gone from the source folder, just not in kmail. workaround exists: quit kmail / kontact, restart the akonadi server, then run akonadictl fsck and akonadictl vacuum. and yes I DO CONSIDER THIS A GRAVE BUG.
I can confirm this here. Testing Fedora 26 which has the same version of Kmail and I am also using IMAP. Restarting akonadi clears it for me. Its a tough one as it does not happen all of the time on this box. Maybe related to this long standing bug? https://bugs.kde.org/show_bug.cgi?id=317933
I'm not sure about that, this one is more like the total opposite, the message content disappears from the preview area but the list stays what it was.
*** This bug has been marked as a duplicate of bug 380824 ***