looks like an error.
i will check this
i have deleted these two lines and replaced with:
defineTumb("UC_DOWN_C",2, 3099, 131, 1, {0,1}, nil, false, "Undercarriage", "Undercarriage U/C Indicator Cut-Off Toggle")
you can do that for yourself. After editing, start a mission and jump in a plane to update the json files. than everything should work as intended.
Fix will be in the next release of BIOS with an rework for the Spitfire.lua.