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@planetf1 already filed a bug report here.

thinking of doing a clean install via usb from beta 7 and restoring from a time machine backup. might waste an afternoon doing so but at least I will be able to use my external monitor until this is resolved in the next beta hopefully.
 
Come on Apple...I know user beware running a beta, but I've been running this on my daily driver since beta 3 without any issue after testing it on my other machine. Now all of sudden my workflow is killed because I can't use my two thunderbolt displays. How is there this big of an oversight in a beta this far in.
 
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Have you folks with the issue tried turning off "Low Power Mode"❓

TinyGrab Screen Shot 9-30-21, 10.39.02 AM.png


Lou
 
My 2013 Mac Pro's MiniDisplayPort to Dell 2718Q stopped working correctly after Beta 8... HDMI worked, but that computer only has HDMI 1.4 so no 4k/60 thru HDMI. I've reported thru the feedback app and reinstalled Big Sur to continue to use this computer correctly.
 
For what it’s worth, by closing the lid with the external monitor attached, then opening the lid, briefly above the keyboard, it shows that the MacBook is connected to the monitor, asking if I would like to Mirror or Extend my main display. This lasts for about 1 second, then disappears, and does not respond by pressing the buttons. The point is, the monitor is connected and the MacBook knows it is connected, however the OS is not coded to execute the followthrough processes for completing the connection.

I’ve tried many work-arounds, and none have worked. I’m about to restore a backup of Beta 7 to get my monitor working again, which is crucial to viewing the information I need to see for my work.
 

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Same issue on my 2013 Trashcan Mac Pro. Beta 8 killed my Thunderbolt monitor (5K ultrafine), only display on this machine. A bit frustating.
 
I have reverted to beta 6 by installing it on top via a bootable usb. only shows beta 8 to update to afterwards. trying to update to beta 7 using the full installer resulted in getting updated to beta 8 and losing the external display again. so I am stuck to beta 6 for a bit here until this get resolved. hopefully in the next beta.
 
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I have tested same Apple TB 27"display on USB-C port on my MacBook Pro, running beta 8 and it works but on iMac still not...
 
Seems at this point limited to Intel Macs. I also had submitted a report to Apple yesterday and seems they already got several similar reports but no response or resolution yet as it seems. I guess at this point there is two choices, revert back to Beta 7 (if you have a backup) or wait and hope there is a Beta 9 next week that fixes it. But that means no external monitor till then …
M1 MBP, same issue
 
MacBook Pro (15-inch, 2016) , Same issues.... :( Sigh.... Well It is a Beta :/
 
imac 2017 . second monitor lg.. it see lg monitor at the system perference : display . Change anything also no change. so the manual mode.
 
Can also confirm that if you can snag a copy of the beta 6 installer, install right over beta 8 that the issue is resolved. Hanging out here until confirmation it's fixed.
 
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MBP 2016 15" same issue avec multiples displays (4 different brand, resolution and adapters). The funny part is that with my Lenovo T32p-20 monitor it still charges thru the USB-C and still send audio to my speakers attached to the monitor.

Feedback report filled.
 
I suggest that MacRumors sends a post on this topic to let the others Beta user that it is not a good idea to install the Beta 8 version. With low adoption rate, Apple might push the investigation a bit more as a priority. Also, I can't imagine engineers are not using external displays... how this could even be release as a beta ?
 
Same issue with MacBook Pro Touchbar 2016 that I was using as a desktop replacement with three monitors (as the internal display failed). Am guessing that the Apple development team are all on M1 devices...
 
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I have exactly the same issue with MacBook Pro Touchbar 15" (mid 2016) after updating to Beta 8 (21A5534d) ! I was using one HP Z27 (4K display) and a Dell E2420HS (FHD) to replace my internal monitor with flashing strips issue. Is Apple forcing everyone who has a internal monitor issue to buy a new laptop....?
 
It appears that beta 8 has killed external monitors. I have read at least 2-3 more people mentioning this in the forums.

Any workarounds to get external monitors to work properly again?
i fixed my issue by plugging in the official charger directly to the macbook, then the screens connected again unplugged and used my adapter to charge the laptop , i own the anker apex hub
 
i fixed my issue by plugging in the official charger directly to the macbook, then the screens connected again unplugged and used my adapter to charge the laptop , i own the anker apex hub
I attempted to do this but it was not effective for me. From time to time I can get the display to work for a few seconds but it does not last as WindowServer always crashes for me.
 
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I have two monitors - one works, one doesn't. The one that works hadn't been connected it before (took it from another room) but oddly they are nearly identical. Same res/brand etc just one is a year later model. Could be a minor spec difference, or could be the fact one hasn't been seen before. Odd, but one consistently works, one consistently doesn't....
 
I have reverted to beta 6 by installing it on top via a bootable usb. only shows beta 8 to update to afterwards. trying to update to beta 7 using the full installer resulted in getting updated to beta 8 and losing the external display again. so I am stuck to beta 6 for a bit here until this get resolved. hopefully in the next beta.
You got a source for the download?
 
Mac Pro 2013 with Displayport displays: The displays connected through DisplayPort no longer work, only HDMI does still work. The display either do not show anything or freeze in the middle of boot up.
 
my head headache cause of this. seem need to back apple tv and extends this like old time.
 
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