Just curious why this is? Usually, keyboard combos on the guest do not cause the host to perform functions. For instance, Win+L on a Windows guest will lock the guest but not the host
But Win+Space does work on both. This is the default keyboard shortcut on Windows 10 for switching between input languages/keyboard layouts.
On Kubuntu, if I set that as the keyboard combo for switching layouts, inputting Win+Space on the guest causes the keyboard layout to change on both guest and host simultaneously.