I don't know why the network applet does this, but on LMDE it "loses track of" available connections even without the screensaver kicking in. They just disappear from the list of networks after a short period of time.
Same as you, if I disable "wireless" and re-enable it refreshes the list and they all come back again. There's no fix that I'm aware of, but a workaround if you want to install Wicd or Wifi Radar. These little extra packages don't seem prone to losing track of available access points like the built in nm applet.
I've more or less just learned to live with it and do the manual "disable/enable" refresh method if I need to.
KDE/Cinnamon/W8.1 - Asus MB, AMD FX-8370 CPU, AMD Radeon R7, 120GB SSD, 2x1TB HDD, 16GB RAM, etc... Please add SOLVED to the Subject if it's resolved.