Land Rover on any of the later vehicles use plastic for their radiator plugs. Plastic is not a bad thing but the way they are manufactured is. If the bung was a solid plastic plug they would not fail but to save a few cents they make them with a hollow centre. What happens here is the radiator water rises up the centre of the plastic cap pushing directly on the top, which is only 2-3mm thick. What makes this worse when the plug is tightened the seal around the top is also trying to break the top off. After a while the cap separates and the water is lost.
We know of many engines that have been severely damaged due to this and some have been destroyed. Land Rovers solution to this is the plug is a service item and should be changed, but this is only patching the problem. Les Richmond Automotive sells a solid brass radiator plug, which never has to be replaced, and is impossible to break. Check your car and if it has a plastic plug “Change it!”