Using Classic Outlook on Windows for personal email, calendar, and contact management
The Outlook In App Team has submitted a bug for this issue and it is currently being investigated.
Much of the original guidance for these kinds of issues is still useful, 32-bit Outlook interface elements unexpectedly render in black, white, or blank - Microsoft Tech Community, and Office apps appear the wrong size or blurry on external monitors (microsoft.com).
Here are things you can check to help mitigate as much as possible.
Main things to check:
- Office is on the latest build
- Office is installed as 64Bit (File, Office Account, About, at the very top of the dialog)
- Update video drivers to the latest provided by the video card manufacturer
- Switch Office into Optimize for compatibility mode. Go to File > Options > General and in the User Interface Options
If you are on Office Subscription and want to get alerted on bug fix status put a machine on Office Insider Beta and submit an In App ticket (Help, Contact Support). This MVP article has some steps in Method 2/3 to also easily switch to Office Insider, Switching between Microsoft 365 Apps for Business/Enterprise, Current, Preview and Beta Channel (Office Insider) without re-installing - MSOutlook.info.