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.
Hopefully the above helps.