On US and UK keyboards in particular that key and the one next to it are the + and - keys.
I've seen other examples of things not quite working correctly (for example there is a shortcut for code blocks which is ctrl + ~ which I have had no luck with because on a UK keyboard that character is on a completely different key). It mostly seems like it is an upstream electron thing but I'm not sure if it has been fully investigated yet.
Ctrl + + / - is a pretty standard shortcut for browsers and applications for zoom in/out.
I know its not a solution but its some background info that might help explain it.
Interestingly there is something odd going on here even on uk keyboards.
Ctrl + + / - works for zoom in/out but Ctrl + Shift + + also zooms in but Ctrl + Shift + - does not zoom out again (which is what you are seeing).