I went with a proposed update on 2019-04-12. After a reboot, I could not login with plasma . If I type my password, the system freezes (mouse and keyboard do not respond , so it's impossible to do a clean shutdown. I have to turn the power off . If I do not type a password, the system answers ''connection failed''. By using plasma-wayland instead, I can login.
From there the system works, but with some problems.
1 - Rawtherapee (both 5.6-RC1 appimage or 5.5 from ppa) and Libreoffice (6.2 from ppa) do not have the minimize - restore - close buttons anymore. I have to hover on their panel tabs to close them.
2 - Artha and Zim Wiki hijack the menu button (top left corner of the panel) or do not launch from their panel icons.
There was another update today (2014-04-15), but nothing has changed regarding these above matters.
Has anybody experienced this situation ?

I wonder if GTK client-side decorations is interfering or something. Check in System Settings -> Window Management -> KWin Scripts if you have a script called something like Enforces Window Decorations on GTK+ window. If so toggle it, if not hit Get New Scripts and download it. See if that makes a difference. (no guarantee as I didn't think libreoffice used CSD; but it's something you could try).
There are also a few widgets which can be (downloaded and) used to add the window controls to the panel if that works for you.