simple LTM virtual server resets client connection but not send request to web servers in the pool

I am trying BIGIP VE 12.1.3 and configured a very simple virtual server with standard type with destination ip at the same subnet of the mgmt ip , service port is HTTP (80), all other settings are the default and it uses an application pool created earlier in which it contains three simple python http servers listening to port 8000. After creation, from the network map I can see the VS and pool and all pool members are showing GREEN (enabled, active, reachable)

I can access the web servers by pointing my browser to the VS IP after the configuration.

But after some time, may be half hour, when I refresh from the browser, I always get connection reset error.

checking the LTM and all of them still showing good status.

I have tried to delete all of them and recreate from the scratch and some times it would work for a while but got the same issue again. Some other times I didn't even get them working at all then I had to delete and recreate again.

I am very confused about this type of behavior

Can anybody share some hints? Thanks