Spent a while chasing a genuinely strange iOS bug while building Aiden (a physical agent that drives phones over USB HID): modifier-key shortcuts like Cmd+V would silently fail while plain keystrokes worked fine every time.
Turned out iOS was routing the command to the wrong process (SpringBoard, not the actual foreground app) whenever a keyboard and mouse were both present at the same time as AssistiveTouch. Confirmed it wasn't specific to our hardware, reproduced it on a completely unrelated gaming keyboard and a Bluetooth keyboard too.