I too see same issues and one of customer deployments this is a major issue with all new iphone users, android and ipad/mac-book works perfectly fine. We did some packet captures and what we found was iphone with 12.X IOS versions have this behavior, they will connect fine and do a request out for captive portal detection and then after the response they will just keep sending 802.11 NULL frames to the WiFi router and after 45-60 seconds it will start sending DNS query for the given captive portal domain and then the auto sign up sheet pops up, We did many rounds of test and packet capture and this behavior is very consistent on iphone X running 12.X IOS versions. We had one iphone 5c running older IOS version and it just works perfectly fine along with IPAD and MAC Books.
Packet Analysis with iphone10X running IOS 12.X version:
802.11 association is complete with timestamp of 3.3 seconds
There is DNS query for captive.apple.com at 3.5 seconds
There is GET request from CNA browser for HTTP GET /hotspot-detect.html. One can see the User-Agent field here "CaptiveNetworkSupport-355.200.27 wispr" which is used by this CNA browser AKA auto sign up application by IOS devices. This is at 3.8 seconds of timestamp.
At 4.09 seconds the WiFi router has responded with HTTP 307 redirect code with complete URL of the captive portal.
After this iphone keeps sending NULL data frames to the WiFi router for which it keeps responding and it goes on for next 45 seconds.
At 49.04 seconds of timestamp we see a DNS to www.apple.com for which it gets a response and it sends another GET request for "/" on appple.com IP address.
Finally at 49.28 seconds of timestamp it sends the DNS query for the domain name as used by this captive portal and then finally it does request to this captive portal and the login page comes up. Also this duration is a bit random so it takes somewhere between 45-60 seconds to show that login page in this CNA browser. Till then user has no clue what is going on or he might just assume he had Internet access and tries to use his apps which may complain of no Internet connection.
It will be nice if IOS team can fix this delay in their Captive Portal Network Assit application and push this fix soon.