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.
There is a time for laughing and a time for not laughing. And this is not one of them! ...Chief Inspector Jacques Clouseau
LMDE 64-bit MATE Edition
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