Radio channels does preserve last value if cut off the PPM wire

I’m not sure if this a hardware or firmware problem:
I have a Pixracer FC that is connected to FrSky D8R-XP receiver. All works good but when I disconnect the PPM wire from the receiver then all radio channels in MissinPlanner preserves their latest values.

Thus I cannot configure Radio failsafe correctly.
My expectation was that radio channels should drop to a minimum values (that are less that working range).

Digging in source code it looks like throttle failsafe will be triggered anyway if there are no updates from radio.

// turn on throttle failsafe if no update from the RC Radio for 500ms or 2000ms if we are using RC_OVERRIDE

Thought I have no armable config to confirm.

Copter docs describes low-throttle failsafe based on current PWM value so it is not very clear that failsafe will be also triggered when no radio input.