It sounds like you're going to have to pull out the sending unit and have a look-see.

Or if you could unplug to the sender wiring back by the tank and measure the resistance between the green/white and brown wires while someone is filling the tank, you might detect a point at which it freezes. Of course you would have to do this with a near empty tank to start with so that you can tell what happens when the fuel level rises up to that 1/5 level.

Would you happen to know if the gauge freezes at the level that the fuel was at when it was stored?