Consistently reproducible on a Dell Latitude 7470 with build $10123.
After a successful installation, unmeasured, the host reboots into the install OpenXT. Upon reboot, the serial output hangs indefinitely with:
Hangs repeatedly until started with flask=permissive|disabled, then no longer happens, even with flask re-enabled.
Install OpenXT 10, non-measured
Reboot into dom0 without hanging.