Hi Matthias,
I have done some tests in my Outlook for Mac 15.6 but cannot reproduce this issue.
Based on my experience, this kind of issue is possibly related to the client or the machine.
For this situation, I suggest you try re-installing the Outlook client to see whether the issue can be resolved. And you can test with a clean boot to see if any other programs are interfering the Outlook for Mac.
Once you get the test results, I highly recommend you post a thread in our Office Forum to get further assistance. Because our Office 365 forum mainly focuses on O365 online services and troubleshooting, we have limited resources for Outlook client or machine issues. The engineers there have more knowledge about client troubleshooting so that they can help you investigate the issue more efficiently. Thanks for your understanding.
In addition, since Outlook for Mac is a newly released product, we are collecting feedback from customers to improve it. You can click the Send-a-Smile button on the upper right corner in Outlook to report everything thing you find to us. We always wants to hear from you and will take your feedback seriously.
Best Regards,
Henry Huang