LE will always prefer to route over Ethernet if Ethernet + WiFi are available but I don't think that's relevant. if your phone connected to the WiFi network cannot access the GUI of an app which is on the LE/HTPC device with a valid IP in the WiFi network; the issue is with the app on the LE/HTPC device not "listening" for traffic on that interface. Apps are "bound" to IP/port combinations (and IP's belong to interfaces). I'd guess the app is bound solely to the Ethernet IP/port and thus does not listen for traffic on the second (valid) WiFi IP/interface on the host system. That's an app issue, not an LE issue.
"netstat -a" will show you all the open ports and assocaited interfaces. Start there.