PhpStorm's KDE keymap uses shortcuts like Alt+1 to display various window panes in the application. These don't work for some reason.

A different shortcut that uses the alt key, Alt+Home, works fine.

I have gone to Global Shortcuts and attempted to assign Alt+1 to an action (just to see if it would warn me about it already being assigned elsewhere), but it worked fine. It doesn't seem to be conflicting. I'm not sure what could be intercepting the key before it is processed. PhpStorm is a Java application, and I believe they bundle their own JRE. I guess I will also open an issue with them.