Second monitor not coming on after sleep

I have a similar issue. MBPro 16" (as Display 3), Apple Thunderbolt Display As Display 2 via USBC to Thunderbolt adapter, and LG Ultrafine as Display 1 via USB thunderbolt cable. If the OS 10.15.2 turns off the displays, when I wake up the system, the Thunderbolt display stays off (even though there are open windows on it). Kludge is to put MacBook back to sleep and re-wake. Displays then come on and behave. Also recently, if the computer times itself to sleep, it crashes the next morning.

Hi. I can confirm that I also have this issue. I have two Apple Thunderbolt displays that are attached to my Macbook Pro 16 via Apple branded Thunderbolt 3 to Thunderbolt 2 adapters. When waking from sleep one or both of the displays will typically (though admittedly not always) fail to wake. The only consistent fix I have found is unplugging and replugging in one of the Thunderbolt 3 to Thunderbolt 2 adapters (i.e. unplug and replug in one of the monitors). Interestingly, it doesn't appear to matter which display (and adapter) is unplugged from the computer; even if I unplug the working monitor (if only one failed to wake) and plug it back in both displays wake.

This is VERY annoying. Apple - please find a fix!

to get a Mac display to become active, you need the Mac to query the display, and the display to answer with its name and capabilities. Otherwise, the display will not be shown as present, and no data will be sent to the display.

This query is only sent at certain times:

• at startup

• at wake from sleep

• at insertion of the Mac-end of the display-cable, provided everything on that cable is ready-to-go

• on invoking Option-(Detect Display) button in Displays preferences

so try doing some of those things and see if the display comes alive.

I thought it might be a solution to try to remember to manually put it to sleep when I get up from the desk. To the contrary, if I do this and do not rewake the computer until the following morning, it will crash. Assume you have seen the other long thread on usb C devices causing the computer (it seems almost any vintage of MacBook Pro) to crash upon wake from sleep? Alas, 13.5.3 does not address the fault.

I also have the problem but the fixes above don't always work. Sometimes the monitor goes off after being on for awhile and having open windows that, of course, I can't see when the monitor is off since the MacBook thinks the monitor is on, but screen is off. Monitor is connected via HDMI through an Apple HDMI to Thunderbolt adaptor. I've tried different HDMI cables - no change. Disconnecting and reconnecting has no effect. Restart doesn't work. Today a Shutdown and Restart did work.

Apple, please fix it!!!!

That may be the case for you. On my two external monitor set up, before the latest OS update to 10.15.4, it was the older Thunderbolt monitor that failed to come back on. Since the update, both monitors come on when waking from sleep IF the MacBook Pro 16" doesn't crash, which it did again this morning. :(

I had a similar issue and contacted apple support. They provided me with a quick fix and I haven't had any issues since. See below:

How to reset the SMC of your Mac

I solved it by not using the two models you mentioned and only using Dell ultra sharp displays with no issues. U2718Q and U3219Q

10.15.4 begins to fix the problem, if the wake is successful both monitors come on, but I still get crashes occasionally when I try to wake from auto-sleep.