SRBs seem to sometimes flame out with a tiny amount of fuel remaining. When this happens, Engine.has_fuel still reads as True and Engine.available_thrust isn't zero. Arguably, one or both of these shouldn't be the case, since the engine flamed out due to lack of fuel (as the right click menu shows).

The check in Engine.HasFuel is whether the fuel is less than 0.001. I'll adjust this to use 0.02 as the threshold, which should fix this case.