Just to say I'm having the same issue with Quantum Plus. It works fine but only for a couple of minutes before logging out, users then being presented with the Captive Portal again to sign back in. It is supposed to keep them signed in for 1 hour (unless they manually sign out). This is causing all sorts of issues, the biggest for staff seems to be completing safeguarding forms as they spend a few minutes filling in the form, only for it not to submit because they've been signed out of Quantum+.
I'm pretty sure the issue has something to do with Safari's cross-site tracking or similar but I have turned off 'Prevent Cross-Site Tracking' and also 'Hide IP Address' and that doesn't seem to have made any difference. Please do let me know if you find a solution! I will do too, I'm also currently in conversation with Exa support.