xscreensaver-auth runs under the uid and gid of the user (not root) in Qubes 4.2. Therefore, the qvm-run command will fail immediately, because it cannot connect to the qubesd socket. /usr/local/bin/fprint-auth needs to be adjusted like that:
#!/bin/sh
exec /usr/bin/sg qubes -c '/usr/bin/qvm-run -q --no-gui -u root sys-usb /usr/bin/fprintd-verify'