Based on my experience with the RV described above, the sensor is not likely to be at fault. My aircraft also has two tanks.
There are three possible causes.
1 the mechanical engine fuel pump is faulty. (my case)
2 Air is able to enter the fuel system before the mechanical engine pump. (unlikely)
3 There is a restriction in the fuel system before the mechanical engine pump. This could take the form of a hose kink, faulty fuel selector, internal hose collapse or just excessively strong springs in the boost pump valves. At low flow on the ground (0.1 LPM?) this will not a problem. When you give it the beans on takeoff and climb, the fuel flow rate jumps to 27lph. Any restrictions, faults, would become evident however with the boost pump on ( assume it is mounted close to the tanks) faults will be masked until switching it off.
Have just read your test flight report today. Suggest you switch off boost pump at 100% power, safe altitude, to ensure engine pump can supply correctly.