Which operating system does the issue occur on?
Linux
If on linux, are you using X11 or Wayland?
X11

It seems like you cannot set keyboard shortcuts to dead keys. I'm using setxkbmap -layout se,us which means that the plus sign + on the american layout is a dead key in the Swedish layout. This in turn means that the key cannot be used as a shortcut key even if I'm currently using the US layout.