Okay. Do you have any pages in your walled garden? Because the way it's supposed to work is, when IOS connects to a WiFi network, it detects the availability of internet access by trying to open certain pages. If it can, it doesn't open a splash page, if not it presents you with the splash page. If the walled garden allows access to those pages, then that technique fails.
Experienced the same problem with the Splash Page showing only for Android devices but not for Apple Devices.
You can try the following:
Checking the Firmware version running on the AP's to see if there is any known bugs - possibly try upgrading the access points.
Check if there MDM software on the IPhones and not on the Androids.
Are you able to run a packet capture when the device try to connect?
Maybe check the wireless health under the wireless - monitor tab to see the number of failed connections and the reason for them. - Might be difficult as the devices are not getting to the splash page to even authenticate.
Review any firewall rules which could be preventing connectivity to cisco specific splash page. - If there was a rule blocking it I would expect it to impact both IPhone and Android.
We never managed to get a resolution for our issue as we migrated to a PSK instead.