After awaiking from the sleep mode, system doesn't make any attempts to reconnectto wi-fi. Unplugging wi-fi' USB cable and plugging it back required every time to establish lost connection. No other way to initiate the reconnection.
I encounter this from time to time after sleep. One workaround that works for me is to use the command line tool nmcli to bring up the connection. Afterwards the nm-applet works fine again. At times waiting for about 3-5 minutes also solves the issue.
This is in my opinion a bug in NetworkManager, we show only connections which are visible by NetworkManager. Can you try to run "nmcli device wifi list" if there are any at the time when you don't see them in applet?
Indeed it is my workaround to run nmcli --ask c up SSID which connects to the given network. Afterwards the connection appears in nm-applet.
Then it really looks like a problem in NetworkManager, which doesn't expose available networks on DBus. Open please a bug for NetworkManager instead. They have moved their project to GitLab if I'm not mistaken.