Yes this is expected behavior on IOS and a design decision for iPhone. If you open another app, like SMS or E-mail, you can't get back to the captive portal login.
I don't know the optimal solution, but one could be to prevent the captive portal popup, then let users open Safari (or other browser) to process the registration. It may work with a 'dummy login' from the popup (maybe for iOS devices only), then redirect to the registration page which then opens in a full browser where people can switch back and forth between mail/SMS/registration process. I don't have an iPhone myself, so can't really test this; but others may have tested and comment below what is a working/optimal solution.
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Herman Robers
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If you have urgent issues, always contact your Aruba partner, distributor, or Aruba TAC Support. Check
https://www.arubanetworks.com/support-services/contact-support/ for how to contact Aruba TAC. Any opinions expressed here are solely my own and not necessarily that of Hewlett Packard Enterprise or Aruba Networks.
In case your problem is solved, please invest the time to post a follow-up with the information on how you solved it. Others can benefit from that.
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