Problems with android phones connecting to captive portal

This is a shot in the dark but I installed captive portal 10 for a library and everyone seems to be able to connect except for android users. It seems as though the phone will connect to the wifi, obtain and IP address, but when I go to get on the internet, I don't get the captive portal agreement page but instead get the webpage I requested. When I go back to the wireless settings on the phone, I notice that the phone is disconnected from wireless.

The only way I am able to get the device connected is if I disable the cellular data on the phone and then connect to the wireless. I'm curious if there is something in android devices that detects that it cannot get out on the internet through the wifi and just disconnects and jumps on cellular data instead. I've had the same results on two android phones that were the same model (samsung galaxy 3).

Typically users don't connect to our wifi with their phones since they do have cellular data so this isn't a huge issue but it is peculiar. I was just curious if anyone else has experienced similar results.

For reference our setup is a wireless access point (was a 3com, just replaced by a Ubiquiti UniFi AP) with no password, connected through the untangle using bridge mode in the untangle networking, and IP addresses are given out by a sonicwall router.

thanks.