I am not able to replicate this issue but according to this documentation, For an event the id property is composite value based on other properties and this value changes when the item is moved from one container (such as a folder or calendar) to another. That might be the case in your scenario.
I would recommend you to raise a support case with Microsoft Graph for more clarity on this, a Support Engineer will be able to assist you better. You can raise support ticket from http://aad.portal.azure.com/ or https://admin.microsoft.com/#/support/requests.
If the response is helpful, please click "Accept Answer" and upvote it. If you have extra questions about this answer, please click "Comment".