Hope you're doing well.
I'm unable to reproduce this behavior. I'm using Weblink generated from the response of Get events Graph API & using this URL I'm able to open the event in Outlook on the web.
Please find below screenshots for better understanding.
Note: Outlook on the web opens the event in the browser if you are signed in to your mailbox. Otherwise, Outlook on the web prompts you to sign in. This URL cannot be accessed from within an iFrame.
Alternatively, you can raise a support case with Microsoft Graph, where a Support Engineer will be able to assist you better.
For raising a ticket, please follow- http://aad.portal.azure.com/ or https://admin.microsoft.com/#/support/requests.
Hope this helps.
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