OS XI Capitan 10.11.6 Wifi cache ip address when AP detect the mac left the session

we had a very weird situation that I had my macbook air with os XI capitan 10.11.6, I can initially connected to wifi with internet services, but somehow left my mac idle around 10 to 20 mins (power saver is set to never). Then I get back to my Mac, I cannot access to internet anymore. But the Wifi still showed I connected wifi service with an valid ip address. I checked with my technical support from network section. they told me my MAC address of my macbook air already left the connection session (not being kicked off by the AP controller). The strange situation I never saw that was my MAC still showing I have valid IP address connected. What I did to fix it by disable and re-enable the wifi service to make the session to establish again.

I don't know if the problem would stem from the OS or not. Is there any one had this experience that I had?