After the upgrade which installed the kde plasma 5.13.4,
I experienced some bugs with the following behavior and more:

• can't add any widget to the panel
• the entire System Tray disappeared except the clock widget
• all the program's widgets in the task bar disappeared also
• text search for installed application not working

In my opinion the bug's priority is "high",
because in this situation KDE is more or less unusable.
Anyway it is not of your direct competence, because the bug is in Debian release,
but maybe you could exercise some sort of pressure on the Debian maintainer staff
to fix this issue the more possible quickly.
Thank you and best regards