Hello DavidBryant,
You need to execute Advanced troubleshooting 802.1X authentication in order to overcome the prompt issue.
This troubleshooting technique applies to any scenario in which wireless or wired connections with 802.1X authentication are attempted and then fail to establish. The workflow covers Windows 7 through Windows 10 for clients, and Windows Server 2008 R2 through Windows Server 2012 R2 for NPS.
Do follow up the below link to execute the troubleshooting.
Hope this answers all your queries, if not please do repost back.
If an Answer is helpful, please click "Accept Answer" and upvote it : )
& also do find the NPS Authentication Status using the below link to execute the troubleshooting
Regards,