Plasma 5 timer widget counts down too fast (goes down 2 seconds every second)

I just upgraded from Kubuntu 17.10 to 18.04.
The timer widget on my plasma desktop now counts down twice as fast as before, counting down 2 seconds every 1 second.
The displayed time goes down as follows:
24:50, 24:48, 24:46, 24:44...

System info:
============
$ lsb_release -a
No LSB modules are available.
Distributor ID: Ubuntu
Description: Ubuntu 18.04 LTS
Release: 18.04
Codename: bionic
$ uname -a
Linux iki.een 4.15.0-20-generic #21-Ubuntu SMP Tue Apr 24 06:16:15 UTC 2018 x86_64 x86_64 x86_64 GNU/Linux
$ dpkg -l plasma-widgets-addons
ii plasma-widgets-addons 4:5.12.4-0ubuntu1 amd64 additional widgets for Plasma 5
$ plasmashell --version
plasmashell 5.12.4
$ kf5-config --version
Qt: 5.9.5
KDE Frameworks: 5.44.0
kf5-config: 1.0