My actual dock was bad / had gone bad at around the same time the update started messing with the displays. After another clean install of the latest v8, it exhibited extremely weird symptoms:
- When I looked in Screen Resolution settings, I saw all three monitors (laptop + 2 external) showing but neither external screen had anything showing on it;
- Using Identify monitors only showed a number on the main display;
- Occasionally one of the external monitors (the one going through DVI) would show up and display my default background wallpaper;
- Running Identify monitors then would still only result in monitor 1 being identified.
Now, mind you, as I said, it showed that it had detected all three monitors, even monitor type for the two external monitors - but there was no interaction between the dock and monitor, and thus no output.
So, I swapped out with another dock, and it went through another driver installation routine, rebooted, and everything is working again.
Thank you, DL team.