Hi @Paul Manning
As far as I know, this is due to Apple keychain restrictions. User certificate is first installed in the Apple keychain store and then expected to be used for authentication.
Currently only Apple-provided apps such as Safari or Mail are parts of the Apple keychain access group and have access to installed certificates stored in keychain, while this functionality is not available in other browsers such as Edge.
Due to the amount of architectural change and Apple cooperation required, the support for client authentication certificates in Edge iOS is not available now. I suggest that you can provide feedback to Edge team through the feedback option in the ellipsis menu in Edge iOS.
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Regards,
Yu Zhou