@Andy J Ludovic Dabee Thank you for your patience and sharing the additional details. After investigation, we found this is a bug on the service side. Issue has been fixed and roll out is expected to be around 1-2 weeks would recommend you track update in https://github.com/Azure/API-Management/releases.
We apologize for the inconvenience caused by the issue. Feel free to add a comment if any questions.
Please accept as Yes if the answer provided is useful, so that you can help others in the community looking for remediation for similar issues.