Phones will put their wifi radio to sleep when there is no activity. This can lead to the AP being unable to handshake with the phone and making a "missing ACK" disconnect which is normal.
It should not occur if the phone is constantly transferring data and there is a good signal strength on the connection.
It happens all the time, after about 5/10 seconds being connected. If I run a speedtest for example, instant drop. I'm right next to the AP now, so signal is very good.
I've also tried with other wireless devices. Same result.
You are right. Apologies for that. It went from me thinking it was my fault for the problem, config is wrong, to thinking it may be a driver/firmware bug. That's all.