It's definitely a bug. I used to have CTRL+X, it's not working anymore. Every time I press this (or any other) combination, only the enpass extension settings getting opened.
The only combination that works correctly is the default CTRL+SHIFT+PERIOD one. First, who designed this combination? In the middle of the keyboard and you have to lift up your hand if you using your right hand for the mouse. On the left side is more natural for the most users and don't need to remove your hand from the mouse (Something like CTRL+SHIFT+A, CTRL+ALT+A, CTRL+SHIFT+F, CTRL+ALT+F).
I'm using Linux (debian buster) / Firefox ESR (up-to-date)