MattF,
You are saying the same device, and then you are saying device(s). Which specific device behaves differently? Again, on the Apple side it is very well documented why this happens. On Android, it is hit and miss based on the device. Apples use the CNA mechanism, androids use something different and cannot be counted on for all devices...
If you are witnessing the behavior with an Apple device, is is either because of the Bypass CNA functionality in the Captive Portal Authentication Profile or the same CNA functionality via a URL extension in ClearPass. If it is with an Android device, there is no way to know what prompts it.