Screens briefly disconnect when unlocking system after installing W10:FCU
 

This began after I upgraded to W10 Pro Fall Creators Update.

1) I lock my computer and leave for a minute.
2) Windows turns the screen off. (This doesn't happen if I lock my PC and immediately unlock it before the screen turns off.)
3) I touch my keyboard to wake the system so I can enter my password.
4) The password screen initially appears on an external monitor (set as primary display) plugged into the dock.
5) The password screen appears on my laptop screen. It's still present on the docked monitor, like when the dock is disconnected from the pc. Any input goes to the screen on the laptop only.
6) The laptop screen goes black again and the active password box returns to the docked monitor.
7) After login all of my applications have been moved to my primary monitor.

This is the same thing that happens if while my computer is active I unplug the dock from my laptop, wait for Windows to redraw all my apps on the laptop screen and then plug the dock back in; which is why I've concluded that the dock is being dis/reconnected when the screen is woke up.

Prior to installing FCU the dock remained connected when I locked my PC and stepped away for a few minutes and my window layout was not disrupted.

In an attempt to resolve the issue I upgraded from the previous driver version I was using (unknown, but probably the latest version as of may 2017) to the latest 8.3 release and then the 8.4 alpha. Neither resolved the issue.

Thank you for the report.
This sounds like a new OS bug as it's new with this new version of Windows and not linked to a driver change.

Is the machine on battery or working from with external power?
The difference between 1 and 2 seems power saving. What kind of machine?

I personally run build 17025 and haven't observed that.

Kind regards,
Alban

External power.

My laptop is a Lenovo YOGO 900-13ISK.