Hi @Jon Harrison,
I am sorry that I wasn't able to test your issue as our team is mainly responsible for SharePoint issues and lacks a test environment for Outlook.
But I found a thread on how to copy a calendar from Public Folder to SharePoint. You can see if there is a problem with your copying method.
If this didn't solve your problem, I suggest you open a ticket and ask.
Please accept my sincerely apologize for any in convenience this may cause. Thank you for your kind understanding.
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