Hi Mike & Cedric,
I understand the saerch feature works well on Outlook Web App, and the issue only happens on Outlook for Mac clients.
So we can see the Online service is working well and this problem is related to the Outlook for Mac clients. I recommend you contact our Outlook for Mac support for the most professional help, so the issue can be fixed more quickly. Thanks for your understanding.
Best Regards,
Vivian Wang