SUMMARY
Left click on desktop background no longer works. Regression from previous version of plasma installed.

STEPS TO REPRODUCE
1. Add left click mouse action on desktop configuration (doesn't exist by default)
2. Apply
3. Test

OBSERVED RESULT
Left click on desktop background have no effect whatever the attached action. 

EXPECTED RESULT
Left click on desktop background rise the expected applet

SOFTWARE/OS VERSIONS
Windows: 
macOS: 
Linux/KDE Plasma: 
(available in About System)
KDE Plasma Version: 5.17.5
KDE Frameworks Version: 5.62.0
Qt Version: 5.12.5

ADDITIONAL INFORMATION
Please note if the same action is both attached to left button and another button (middle button for example), action attached to the other button works fine.

Relaunching plasma environment (kquitapp5 plasmashell && kstart5 plasmashell) have no effect.

Here is the plasma-org.kde.plasma.desktop-appletsrc file illustrating that:

[ActionPlugins][0]
LeftButton;NoModifier=org.kde.applauncher
MidButton;NoModifier=org.kde.applauncher
RightButton;NoModifier=org.kde.contextmenu
wheel:Vertical;NoModifier=org.kde.switchdesktop