@Alistair Russell, Thanks for posting in Q&A.
From your description, we know that when you sign in to a device in kiosk mode with multiple Azure AD users, you can't set the device to kiosk mode.
For the issue, I did the same test as your configuration and encountered the same error message, it may be that the xml file you deployed is not correctly recognized by the CSP. We recommend that you open case follow the link below for more help.
Get support in the Microsoft Intune admin center - Microsoft Intune | Microsoft Learn
Thanks for your kind understanding.
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