Hi David,
The reason is the design in Identity Awareness side.
Once the gateway opens/redirects the browser to the captive portal, the PDP is "waiting" for the end user's input for credentials. Once this flow is triggered, we are clearing the current session saved for this IP, and creates a placeholder for the new credentials to be received. It means, even if the user will not enter credentials in the portal, the fact that this client IP initiate a traffic to the portal, clears the existing session.
I will add and say, that in case you have configured an automatic redirection to the portal, this redirection will happen only if the PDP doesn't have any information for the end user IP.
I hope it helps.
Thanks,
Royi Priov
R&D Group manager, Infinity Identity