I don't know the specifics of your FJR but the overflow hose from radiator cap area correctly goes to the bottom of the reservoir no matter the water cooled vehicle. This hose blows and sucks. Hot steam has to bubble through cooler liquid in the reservoir. When engine cools liquid is sucked back into the radiator.
Ok so mine is connected to the bottom, however, the spout it's attached to is totally sealed up. It looks like it melted. And no fluid will pass through. So at about 4 bars on the gauge it starts to lose fluid around that hose and then runs hot. Gets up to the last bar flashing.
2017 Royal Enfield Continental GT 535
I ride bikes cause four wheels are much harder to lean🤪