I have wasted many hours on this problem. It relates to several deficiencies both in documentation of the Si4463 and the example Arduino code which has been widely distributed.
I would have used the term open drain or open collector to describe the nIRQ output from the Si4463. This advises me that a pull up resistor or source is required.
I have also still not been able to find any clear documentation of the modes and bit patterns required to setup the GPIO pins
In the sample Arduino code I would have setup the pin 2 to have a pull up. This means modifying the distributed Arduino library code so that the Arduino provides a pull up
//init slave select pin
digitalWrite(_nSELPin, INPUT_PULLUP); //Replaced HIGH with INPUT_PULLUP
//No other change needed
//Tested and works fine without pullup resistor JIB 20191010 1532 Au time
// depends on RF4463 spi timing
// too fast may cause error