I filmed my computer boot up to better show what happens. The right monitor is on the integrated video card (the R7), and doesn't show anything initially. The center and left monitor turn on and mirror each other during boot, and show grub and most of the kernel and systemd messages, then at some point all output is sent to the right monitor. I believe the systemd log says something along the lines of "detecting all video cards" at that point.
** Attachment added: "Computerbootup.webm" https://bugs.launchpad.net/ubuntu/+source/lightdm/+bug/1692437/+attachment/5235238/+files/Computerbootup.webm -- You received this bug notification because you are a member of Ubuntu Touch seeded packages, which is subscribed to lightdm in Ubuntu. https://bugs.launchpad.net/bugs/1692437 Title: Monitors on second GPU not working after upgrade to zesty Status in Light Display Manager: New Status in xserver-xorg-driver-radeonhd: Invalid Status in lightdm package in Ubuntu: New Bug description: I'm not 100% sure this is the right package to file the bug against, but since upgrading to zesty the two monitors on my second GPU are no longer detected once I reach the greeter screen at login time. However, both of them display information during boot. Let me know how I can narrow down the bug report. ProblemType: Bug DistroRelease: Ubuntu 17.04 Package: lightdm 1.22.0-0ubuntu2.1 ProcVersionSignature: Ubuntu 4.8.0-52.55-generic 4.8.17 Uname: Linux 4.8.0-52-generic x86_64 ApportVersion: 2.20.4-0ubuntu4 Architecture: amd64 CurrentDesktop: GNOME Date: Mon May 22 01:42:58 2017 EcryptfsInUse: Yes InstallationDate: Installed on 2016-04-29 (388 days ago) InstallationMedia: Ubuntu 16.04 LTS "Xenial Xerus" - Release amd64 (20160420.1) SourcePackage: lightdm UpgradeStatus: Upgraded to zesty on 2017-05-22 (0 days ago) To manage notifications about this bug go to: https://bugs.launchpad.net/lightdm/+bug/1692437/+subscriptions -- Mailing list: https://launchpad.net/~touch-packages Post to : email@example.com Unsubscribe : https://launchpad.net/~touch-packages More help : https://help.launchpad.net/ListHelp