Same problem here...
My bike: 2009 XB12Scg.
Bike is charging at idle, but cuts charging at 2500@ and higher. But only sometimes, not always but over time, more often than not, causing battery to discharge.
I have checked all grounds, including the weak one behind the triple tree. All fine.
I have checked all wires and connections. And they're all in good condition (I cleaned all connectors too - they're like new).
I have measured the AC voltage with multimeter, and it shows 30V at idle, and climbs to 72V at 4000@.
I have measured the resistance between stator output pins to 6 ohms, and tested NO connection between each output pin and ground.
I have measured unloaded battery voltage to 12,6V, and 12,2V with ignition turned on (engine off).
With these tests done, I concluded that my voltage regulator must be faulty, although working at low AC input (30V AC), when idling.
Kind regards
Sebastian