2020 27" iMac display gets half opaque when shutting down computer

Boot into Safe Mode by booting with the Shift key held down and check there to see if the problem persists.  Reboot normally and test again.

NOTE: Safe Mode boot can take up to 10 minutes as it's doing the following; 

• Verifies your startup disk and attempts to repair directory issues, if needed

• Loads only required kernel extensions (prevents 3rd party kernel/extensions from loading)

• Prevents Startup Items and Login Items from opening automatically

• Disables user-installed fonts 

• Deletes font caches, kernel cache, and other system cache files

So, i had the same issue. Contacted Apple, couldn't give me any proper answer to the issue and i have been told that they didn't know about it internally??? I also sended them the link of this discussion. So new computer, fresh out of the box, installed adobe pack and some standard stuff and i noticed the half screen issue at shutdown. It flickers 1 time, then shows half opaque exactly in the middle before actually shutting down.

So, my thougth where this. A: or it is a hardware videocard issue. B: it's iOs issue so software... my bet would be B because it is only happening at shutdown. A defect videocard would probarbly also show other issues during usage of the imac.

They couldn't give me a proper answer and told me to return the iMac... fine, it would be the second one i would be returning so i was not in a hurry to do so because i must de-install everything on it (again) and will cost me a lot of time.

But.... and here it comes.... i decided to wait for next iOs update... and today there was one... and to my surprise... the issue seems to be gone!!!

So, i am very curious if this is also the case with some of you guys? so please update and see if it is indeed fixed as it seems to be here. Big Sur 11.2.1

Hope to hear from somebody soon.

I spent time talking with apple support about my iMac with the exact came screen shutdown problem.

After reinstalling the OS the problem remained so they felt it was a hardware issue and the machine has now been takeaway for repair. I'll post again once I have the repair feedback.

It might be useful to point out that this problem can go away after a PRAM reset and shutting down after a short time. However, the problem has returned each time after shutting down after a longer/normal use time.

Give this a try: boot into Safe Mode according to Start up your Mac in safe mode - Apple Support and see if the problem persists. Reboot normally and check again.

NOTE: Safe Mode boot can take up to 10 minutes as it's doing the following; 

• Verifies your startup disk and attempts to repair directory issues, if needed

• Loads only required kernel extensions (prevents 3rd party kernel/extensions from loading)

• Prevents Startup Items and Login Items from opening automatically

• Disables user-installed fonts 

• Deletes font caches, kernel cache, and other system cache files

Hi, after many tests, I found that the only way to avoid the problem is to disable the True Tone…

Download and run Etrecheck. Etrecheck is a diagnostic tool that was developed by one of the most respected users here in the ASC and recommended by Apple Support  to provide a snapshot of the system and help identify the more obvious culprits that can adversely affect a Mac's performance.

IMPORTANT:

Before running Etrecheck assign Full Disk Access to Etrecheck in the Etrecheck's Privacy preference pane so that it can get additional information from the Console and log files for the report:

Also click and read the About info to further permit full disk access.

Copy the report

and use the Additional Text button to include the report in your reply.

Then we can examine the report and see if we can determine what might be causing the problem.

Download and run Etrecheck. Etrecheck is a diagnostic tool that was developed by one of the most respected users here in the ASC and recommended by Apple Support  to provide a snapshot of the system and help identify the more obvious culprits that can adversely affect a Mac's performance.

IMPORTANT:

Before running Etrecheck assign Full Disk Access to Etrecheck in the Etrecheck's Privacy preference pane so that it can get additional information from the Console and log files for the report:

Also click and read the About info to further permit full disk access.

Copy the report

and use the Additional Text button to include the report in your reply.

Then we can examine the report and see if we can determine what might be causing the problem.

I don't see any of the usual bad actor suspects in the report but there are a couple of items that are unnecessary: ClamXAV and Tunnelblick.

There is no reason to ever install or run any 3rd party "cleaning", "optimizing", "speed-up", anti-virus, VPN or security apps on your Mac.  This user tip describes what you need to know and do in order to protect your Mac: Effective defenses against malware and other threats - Apple Community.  

There are no known viruses, i.e. self propagating, for Macs.  There are, however, adware and malware which require the user to install although unwittingly most of the time thru sneaky links, etc.   

Anti Virus developers try to group all types as viruses into their ad campaigns of fear.  They do a poor job of the detecting and isolating the adware and malware.  Since there are no viruses these apps use up a lot of system resources searching for what is non-existent and adversely affect system and app performance.

There is one app, Malwarebytes, which was developed by a long time contributor to these forums and a highly respected member of the computer security community, that is desshoigned solely to seek out adware and known malware and remove it.  The free version is more than adequate for most users.  

Additionally unless you're using a true VPN tunnel, such as between you and your employer's or bank's servers, they are useless from a privacy standpoint: Public VPN's are anything but private.  

I recommend you remove both according to the developer's instructions.

I bought imac 4 days ago. I have had this problem since the first day I got it. 
Had the same problem that had come with Catalina, I updated big sur but still the same problem continues.
 It bothered me a lot and I did a lot of research and experimentation. 
I think it creates a software bug while trying to set the screen night mode. 
Now I've turned off the night mode completely. 
Please turn it off, let's try it together!

Because we need to understand whether it is software or hardware for a return :)

That's telling us that there's some software that's causing the problem, verify likely 3rd party software.

Post an Etrecheck report as discussed previously in this topic. This can rule out any 3rd party software that you have …… or find the culprit.

I got the error again even though nightshift and trutone were turned off.
I believe it's hardware now

This appears to be a software bug in macOS Big Sur.

You can let Apple know about the Bug here: Product Feedback - Apple.

Cheers,

Jack