With latest version of Projucer (5.4.7 build date 10 Feb 2020) on macOS Catalina. I have not tested on any other systems.
Attempting to open a jucer file from Finder fails unless Projucer is already running. The Finder window containing the jucer file loses focus, which appears to indicate that Projucer is about to open, but it does not actually open.
If Projucer is already running then opening the jucer file from the Finder works as I’d expect: focus switches to Projucer and the jucer file is opened.