Azure Certificate Validation Failed By Token Handler

I have two client certificates that I have installed for my API project in the Azure portal. I have set the config option to enable them and can verify that they can both be found in the CurrentUser 'My' store. My problem is now that one of them is failing validation. On my local machine I have this cert installed in the Trusted People store of LocalMachine. Is there anything I can with Azure to make this work?

Is the location of the cert the issue or is it possibly something with the certificate itself?

**Note that this is an app service, not a cloud service