I discovered there is a whole host of sysadmins out there attempting to actively subvert the iOS capitve portal detection. They try to figure out the domains used and whitelist them so iOS will think it is connected to a good network, but they redirect everything else which horribly breaks SSL connections. The whole thing is an arms race where Apple adds new domains to iOS but doesn't start using them until a certain date to evade the whitelist.

The stated reason for this stupidity? The mini-browser that pops up doesn't work with their stupid captive portal login or payment page. Fix the page? Nahhh, let's just fuck everyone's network connections instead. It caused an endless barrage of battery-draining network errors, eating the retry count and eventually causing POST requests to error out. The complete lack of respect for the users, internet protocol standards, etc was extremely evident.

We ultimately stuck a file with a specific phrase in a text file on the site, then when the app was having network trouble the code tries to fetch that file. If it gets an HTML response it marks it as in captive portal mode. IIRC it offered to take the user to safari and would open some non-HTTPS URL so the captive portal redirect could fire and let the user know.

I hope anyone trying to circumvent captive portal detection dies a very painful death, then gets revived, recovers through the miracle of modern medical technology, then dies another painful death.