I've been having the same issue with my Sprint HTC One since the last update and have been searching forever for an answer until someone in here mentioned something that fixed it. You said Wi-Fi Optimization is off - I assume that's under the wifi settings? If you go into Mobile Data > Automatic Connections. Uncheck Wifi Optimizer. When I went back into the setting I got a long message about how it'll automatically connect to the wifi if there is a weak mobile data signal. This worked for me, and again I'm also on Sprint, so hopefully it'll work for you too.