The disposable VM startup mechanism waits for the process to complete for the application being called. Unfortunately this doesn’t work on some Windows applications due to the way they process creating, notably Explorer and Firefox as far as I remember. As a workaround you can run a “Command Line” application or make a disposable VM with a permanent name and use it (like sys-usb and sys-firewall)
qvm-create --disp --template win-dvm-template win-dvm