Hope you're doing well.
FindMeetingTimes Graph API returns the nearest available time to the start time specified in the request. The suggestion will always be at the hour or half-past. Currently this API doesn't support 15 minutes incremental block. I would suggest you to use 30 minutes increment as the meeting duration.
However, you can always 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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