What probably is the issue is that the MAC authentication is hit on the wrong ClearPass service. If you used the wizard, it will create services for MAC authentication as well for MAC caching; if those are not properly configured and the MAC authentication hits the guest user authentication instead, you will get this kind of behavior.
If you want to investigate/learn yourself, I'd suggest that you check out the Guest part in this video series: https://community.arubanetworks.com/t5/Security/Aruba-ClearPass-Workshop-Video-series/td-p/291597
It shows how to setup all parts of the Guest access, including the services; and what you should expect in the logging.
In the case you need a quick fix, contact your Aruba partner or Aruba TAC.