Update: This fault was tracked down to a damaged display port cable. Some of the pins in one of the connectors were bent so badly that I have no idea how it was functioning. A replacement resurrected the affected display.
I can only conclude that there must have been some extra status checking code in one of the updates which introduced a test on the faulty lines. The test returned an erroneous result and triggered the driver to issue an "enter power save" command to that monitor. This bug can be closed - and I can stop attempting to bisect the Xorg code.. ** Changed in: xorg (Ubuntu) Status: New => Invalid -- You received this bug notification because you are a member of Ubuntu Bugs, which is subscribed to Ubuntu. https://bugs.launchpad.net/bugs/1654772 Title: Lost one screen from triple multi monitor radeon setup. To manage notifications about this bug go to: https://bugs.launchpad.net/ubuntu/+source/xorg/+bug/1654772/+subscriptions -- ubuntu-bugs mailing list ubuntu-bugs@lists.ubuntu.com https://lists.ubuntu.com/mailman/listinfo/ubuntu-bugs