Android client has audio live-stream traffic but with request timed out for ping test at the same

[Nelson_N]

I have odd observed with my android device (8.0.0) and AP7632 with 5.9.4.1-004R.

WLAN setting on AP7632.

1.  bridging-mode local
    encryption-type ccmp
    authentication-type none
2. ssid - testwlan
3. others follows the defaults

the ssid testwan is mapped on radio 1. All parameters on radio 1 are set as default.

After my android device connected to this WLAN, the ping test is ok (ping request and reply are observed). Then, opening internet radio apps to listen which the ping test is still continuous.

Several minutes later, my android device screen is going to off. I am still listening the internet radio and ping test is continuous with ping request and reply are observed. 
After several minutes further, ping test is failed with the request timed out message shown on my PC command prompt.it was supposed that my android client should be in offline state. But I can still hear the music from the internet radio apps. It is sure that the mobile data connection on my android device is OFF.

All the settings on APs are default except the configuration of WLAN testwlan. Anyone can provide the recommendation on the settings

Thanks