That's because in guest mode, you are connecting adhoc to the Chromecast. In your Chromecast setup, in the Guest Mode dialogue, you'll see you have different PIN than your "normal" Chromecast mode.
Don't forget that in Guest Mode, any device in with connectivity to the Chromecast can connect adhoc if they know the Guest PIN.
From the Aruba IAP side, I assume your iPhones are in the the same SSID and VLAN as the Chromecast and you have enabled airgroup services, correct?