Comment 3 for bug 1334218

I don't believe this to be a valid issue, rather a preference. Checking the desktop entry spec it states that if an absolute path is used, then that file will be used. The example further below does show a shortened icon value, but this description indicates an absolute path is a valid option:

https://specifications.freedesktop.org/desktop-entry-spec/desktop-entry-spec-latest.html#recognized-keys

Key: Icon
Description: Icon to display in file manager, menus, etc. If the name is an absolute path, the given file will be used. If the name is not an absolute path, the algorithm described in the Icon Theme Specification will be used to locate the icon.