Let's see if someone here might know this issue and hopefully have a fix or work around. I do have a ticket with Microsoft, but it is moving glacial slow...
In the above screenshot you can see that the calendar has not rendered all the way down. I am left with a white space you cannot click in to. Where the cursor is I clicked in that spot and the selection was above the area I clicked. Sometimes the render issue will happen across the length of the calendar window, other times it will happen in a shared calendar window. A sure-fire way to get this issue to show up is to just resize the window with the calendar in view and snap into position, after a couple of attempts like that you cna generally get it to appear.
Initially the issue was showing on an Azure Virtual Machine, but I have since been able to reproduce it on a physical desktop and two different laptops with two different onboard graphics, Intel and AMD. All drivers, Windows and Office is up to date. Re-creating profiles, reinstalling Office, joining the Insider Builds, working with a small to large resolution, all do not get rid of the issue.
A workaround that was working for a short period of time was disabling hardware acceleration but that no longer works. All Office application are now activerly ignoring that registry key.
At this point I am 99% certain it is a rendering engine issue within Outlook/Office but I am really hopeful someone might have a fix for me to try.