Hi Marc,
To troubleshoot the issue, I’d like to confirm the following information:
1. I understand the issue occurred when sending Meeting invite form your Office 365 account to your personal Gmail account. If so, create a calendar event from an Office 365 account, and send an invitation to some internal users, see if the same issue will occur.
2. Do the issue happens to any other users when they sending a meeting request?
3. You mentioned the issue occurs both in the Outlook client and Outlook Web App(https://outlook.office365.com/owa/), please close the Outlook client for some time to test if the issue exists just with OWA. This can help narrow down whether it’s related to the client.
Any updates will be appreciated.
Best regards,
Jamie