Comment 6 for bug 1257048

Thanks for providing the logs, seems this is actually a bug in Unity8.

From your syslog, this is where the problem happens:
Dec 3 14:14:42 ubuntu-phablet powerd[851]: screen power setting failed: GDBus.Error:org.freedesktop.DBus.Error.ServiceUnknown: The name com.canonical.Unity.Screen was not provided by any .service files
Dec 3 14:14:42 ubuntu-phablet powerd[851]: failed to set display power mode, not clearing state

And from powerd:
"""
    case DISPLAY_STATE_OFF:
        if (actual_screen_state == DISPLAY_STATE_OFF) {
            /* Nothing to do */
            return;
        }

        powerd_debug("turning off display");
        if (using_ab)
            powerd_autobrightness_disable();
        powerd_set_brightness(0);
        if (!display_set_power_mode(0, "off")) {
            powerd_warn("failed to set display power mode, not clearing state");
            return;
        }
"""

This basically means that powerd tried to deactivate the display, but the needed Unity8 interface wasn't available at that time (name com.canonical.Unity.Screen). You also said that it works fine after interacting with the display, so I wonder if something changed in Unity8 that made the service to not be necessarily up right after the boot.

The interesting thing here is that I'm unable to reproduce it locally, and tried many times already.