Running Outlook on the iMac causes the screen to flicker and eventually black out and reduce to half/split screen display.

I HAVE THE SAME ISSUE AND TOOK TO APPLE WITH SAME ANSWERS ABOVE - I narrowed it down to OUTLOOK and the update, which is why i searched google and came to this forum - WOW, it really is OUTLOOK - I do not use Skype - MICROSOFT NEEDS TO ADDRESS this  - i do not want to switch to apple mail - !

Reason I bought a 5k iMac is for the fantastic resolution ......Low resolution for me is not the fix -

I think you are misunderstanding what low resolution mode is.  It only affects the single application being used (Outlook), one of your other applications or desktop is affected.  You don't loose any resolution at all.

-Quit the application

-Go to the Applications folder.  

-Select the application you need to run in low resolution mode.  In this case, Outlook.

-Go to "Get Info" You can do this any of three ways (1) Right click and go to "Get Info" (2) Left click the application, click on file, click on "Get Info" (3) Left click the application, press command and "i" on the keyboard

-Toward the top of the window that opens, check the box next to "Low Resolution Mode"

This is a workaround, not a fix, true.  But emails and calendars aren't exactly something I'm clamoring to see on a Retina display.

It's now been a week since I enabled low resolution mode for Outlook and the issue has not returned.

I agree.  Why would I step down my beautiful resolution?  Microsoft needs to fix this.

You are not stepping down your beautiful resolution.  You are essential telling macOS to display Outlook (and only Outlook) in standard pixel density instead of Retina pixel density.  This only affects the specific app you run in low resolution mode.

Apple went out of their way to design a feature that allows apps to run in low resolution mode for retina displays because they know it can cause problems.  This isn't just a Microsoft problem.  Microsoft does need to be a part of the solution since this is happening with their software but not all software, but it happens with non-Microsoft products, as well.  This needs to be a collaboration between Apple, AMD, and Microsoft.

If you want to use another mail and/or calendar client, fine.  But if you work in an enterprise environment like I do and need to provide a solution to an end user that involves Outlook because that's the company approved tool and the most cooperative thing to use with an O365 exchange server, then...

-Quit the application

-Go to the Applications folder.  

-Select the application you need to run in low resolution mode.  In this case, Outlook.

-Go to "Get Info" You can do this any of three ways (1) Right click and go to "Get Info" (2) Left click the application, click on file, click on "Get Info" (3) Left click the application, press command and "i" on the keyboard

-Toward the top of the window that opens, check the box next to "Low Resolution Mode"

Same issue here on my iMac (Retina 5K, 27-inch, Late 2015). Been happening all year.

Brought iMac into Apple Store THREE TIMES, they replaced the Logic Board 2x and the display 1x and the problem persisted.

It seemed to always happen while using Outlook, so I Google'd the issue and here I am.

How is this not fixed?

How is this not fixed?

Because it only seems to affect one model of computer that hasn't been sold since June 2017.  I don't see Microsoft, AMD, and Apple spending the time, money, and other resources to get together and release a software or hardware patch for this issue when there is a workaround.