@Ross Bailey Welcome to Microsoft Q&A Forum, Thank you for posting your query here!
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Please follow the below steps:
Plan 1:
Please check if you can update the System message using Azure Open AI Studio instead of AI Studio
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Plan 2:
Please clear the browser cache and cookies and test again.
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Plan 3:
Please ensure that your account have the Cognitive Services Contributor
RBAC role permissions added to the Azure Open AI Resource from Azure portal--> Azure Open AI Resource --> Access Control IAM.
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Plan 4:
Please click on the F12 button, navigate to the console tab and check if any errors while trying to place cursor on system message.
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Plan 5:
Try from a different browser and check.
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Plan 6:
If none of the above plans helps, please collect the browser HAR traces while reproducing the issue and share the traces with me.
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Hope this helps. If you have any follow-up questions, please let me know. I would be happy to help.