What is a fuel tank float valve and how does it work?
I don't know about the fuel float valve of the F35A. A float valve is basically the principle used in toilet flush: A device that:


senses liquid level in a tank, and
open/close a valve according to this level.


The device may rely on a float acting on a horizontal lever, or it may be a float sliding vertically along a rod.

There are several float valves on this schematics of the Boeing 737 fuel system:


Source

If this doesn't work, the tank may be reported as always full, as for the F35A of your report.