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I have dual LG 4k monitors on a 2015 macbook pro on High Sierra. One is connected via a belkin thunderbolt 2 hub. They both work at 4k 60Hz no problem (via displayports, not HDMI, that is).

The system itself is set to never sleep with Prevent computer from sleeping automatically when the display is off in Energy Saver, in System Preferences, and I have Turn display off after 3hrs rather than never.

When I return after more than 3 hrs and wake, i.e. cause the screens to switch on again (the system I believe should not have actually been in sleep due to power connection remaining on the laptop) the two screens consistently swap their settings with each other, such that the one I have at 270 rotation for vertical aspect swaps with the one I have at normal rotation, and I have to manually swap them back again.

Strange and annoying. Not quite sure what to do.

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    I've had almost identical monitors forget which is which. I 'fixed' it by actually physically swapping them to the layout the Mac seemed to 'prefer'. It's not done it since. YMMV, but might be worth a shot.
    – Tetsujin
    Commented Aug 22, 2019 at 6:38
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    interesting. i tried swapping which is plugged into which of the thunderbolt ports, which made no difference to this issue, but in terms of the mac preferring one of the physical displays (i.e. serial number) to be in one of the orientations, hadn't thought of that - will try it. if this is true then maybe the display with the serial number "it first met" is the one it prefers to be at zero degrees rotation!
    – mwal
    Commented Aug 22, 2019 at 7:16
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    Oh dear - I swapped the physical monitors and still, the mac swaps their orientations when it wakes the displays from sleep (mac itself not sleeping). :( I noticed also that, in terms of which of the two I have the menu bar on, so "primary desktop", desktop and screensaver in control panel reports as expected, but display settings indicates that the monitor with the menu bar, and therefore the "primary" is in fact LG HDR 4k (2), and the non-primary is (1). That was after I switched them. I wonder if there is a cache somewhere re. monitors the system has seen which I can wipe.
    – mwal
    Commented Aug 24, 2019 at 5:45
  • by "as expected" I meant that desktop and screen saver reports the non-primary display, i.e. that which has the menu bar on it, in "arrangement", as "secondary". But when in display settings itself I get "LG HDR 4k (2)" as the name of the primary, which seems odd. That only occurred after swapping the physical screens, so it may suggest that the "(2)" refers to the physical device.
    – mwal
    Commented Aug 24, 2019 at 5:53
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    This is an EDID thing. macOS looks at the EDID info which includes an identifier so that it can say "this monitor is configured like this" and "that monitor is configured like that." Thing is, there needs to be a unique identifier (like an SN#) so it always associated with the correct monitor. LG has a habit of not doing this, so it's a grab bag of what monitor gets assigned what profile. I'm trying to find if there's way to either write "extra data" to the EPROM in the monitor or spoof the EDID data macOS gets from it. My problem is, I don't have identical monitors to test.
    – Allan
    Commented May 24, 2020 at 14:05

2 Answers 2

Reset to default
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This is the combination of:

  • macOS not caring about serial numbers
  • a race condition.

See my detailed answer in Primary display randomly changes

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I solved the problem using a DP cable and an HDMI cable instead of 2 HDMI cables. I now have 2 different EDIDs and AlphanumericSerialNumbers.

See my detailed case and solution here.

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