KMail tends to have one unread mail per (IMAP) folder

KMail version shipping with 18.04 LTS frequently has one stuck unread message in the account/folder pane even though all messages are shown as read in the message list.

This can be temporarily "fixed" by scrolling through the message list with keyboard shortcuts until the one "magically unread" message has been viewed again, but will occur again when new unread mail pops up in that folder.

ProblemType: Bug
DistroRelease: Ubuntu 18.04
Package: kmail 4:17.12.3-0ubuntu1
ProcVersionSignature: Ubuntu 4.15.0-39.42-generic 4.15.18
Uname: Linux 4.15.0-39-generic x86_64
ApportVersion: 2.20.9-0ubuntu7.5
Architecture: amd64
CurrentDesktop: KDE
Date: Tue Nov 20 10:13:50 2018
SourcePackage: kmail
UpgradeStatus: No upgrade log present (probably fresh install)