Ctrl+Alt+Shift shortcuts not working on Linux Mint 19

I migrated from Mint 17.3 MATE to Mint 19 MATE and my Ctrl+Alt+Shift keyboard shortcuts stopped working.

If I try to enter a Ctrl+Alt+Shift shortcut in mate-keybinding-properties, the Alt doesn't register

For example: If I press Ctrl+Alt+Shift+X, it registers as Ctrl+Shift+X.

Is there a way to fix it?

This is an Xorg bug and as I understand it manifests itself by Xorg giving preference to shorter shortcuts before more complex ones that include the same keys as a subset.

There is a patch in the Freedesktop bugzilla ticket and you can install it using the commands below (documented already in this Cinnamon issue on Github):

sudo add-apt-repository ppa:nrbrtx/xorg-hotkeys
sudo apt update
sudo apt dist-upgrade

It is an old bug (check the bugtracker here), and there is a patch:

sudo add-apt-repository ppa:ubuntu-wine/ppa
sudo apt-get update

# install the patch from ppa
sudo apt-get dist-upgrade

Explanation

As I understood, Linux Mint starting from the version 19 uses the updated version of the XOrg-system, which uses the keypress event instead of the old good keyrelease, and now the shortest shortcuts hide the longer ones. This behavior is encouraged by XOrg itself.

The installed patch changes the settings of the XOrg (though, I'm not sure about the last statement).