Does javascript/browsers prevent the detection of a printscreen? (Mac and Windows)

Question

First, I'm working on Mac but it was also tested on a Windows PC. I wrote a small keylogger in Javascript that is listening to keydown and keyup events and manages an array of the currently pressed keys. It works with every combination of keys, except the Mac printscreen combination of Command+Shift+3. It works with 2 in the end and actually every character that is not 3. Is there maybe something preventing from detecting a printscreen?