As far as I'm able to tell, the aircraft don't change back to normal fuel when Krastorio 2's 'Rebalance Vehicle Fuel Usage' setting is toggled off, while vanilla vehicles do. Might I recommend adding a check to your Krastorio compat updates for whether the vanilla car uses K2 fuel or not, and using that to decide whether the aircraft need to be updated? Something like this:

if mods["Krastorio2"] then
if data.raw.car["car"].burner.fuel_category ~= "vehicle-fuel" then
goto fuelskip
else
--Update Fuel to Krastorio 2 vehicle fuel here
end
::fuelskip::
end