Created attachment 126315 [details]
Full Path - With Extension

SUMMARY
When specifying the full path of a svg icon (ie /path/icon.svg), with or without the ".svg" extension specified, for an icon in a .desktop file the icon can't render in either dolphin or in the application launcher. When using the icon in the default icons path the icon displays correctly. 

This results in various applications that uses custom icon paths for svg icons to not display correctly in the application launcher and dolphin.

As test case the following can be done to reproduce (Using KCalc as an example, same problem can be found for JetBrains IDEs and multiple other applications):
1) Copy accessories-calculator.svg from default theme folders to desktop, ie /home/user/desktop.
2) Open /usr/share/applications/org.kde.kcalc.desktop
3) Edit icon path for following three scenarios:
3a) Use full path: icon=/home/user/desktop/accessories-calculator.svg => Result: .desktop file does not display icon.
3b) Use full path excluding extension: icon=/home/user/desktop/accessories-calculator.svg => Result: .desktop file does not display icon.
3c) Use default icon path: icon=accessories-calculator or icon=accessories-calculator.svg  => Result: .desktop file displays icon.

EXPECTED RESULT
Icons should be displayed when full file paths, with the extension specified, are used.

SOFTWARE/OS VERSIONS
Linux/KDE Plasma: OpenSuse TumbleWeed
(available in About System)
KDE Plasma Version: 5.18.0
KDE Frameworks Version: 5.67.0
Qt Version: 5.14.1

ADDITIONAL INFORMATION
Problem seems to be distribution agnostic. Tested on Kubuntu, KDE Neon, Manjaro, OpenSuse and FerenOS