After some upgrades of my Debian Linux "stretch" including sudo apt install -t=stretch-backports evolution network-manager
the bug did not disappeared, but now when network disconnects, there is no more visible indication of the bug in Gnome 3 panel. The connection icon (near the top right corner of the screen) remains active (there are no symptoms of disconnection). Otherwise the bug remains the same: I need to disconnect and connect again about every hour of half for the network to work.
