Had a similar thing - on mine it was simply the factory tabs on the wires connecting to the indicator backplate had loosened over time. Enough to stop one bulb working so the other flashes twice as fast to indicate a missing bulb. Crimped them a bit with pliers, all good.
The factory relay has this fast flashing warning built-in, but it makes using LED indicators a pain as the low current triggers the fast flashing. There is a mod where you prise open the relay, cut pin 7 on the IC chip to force the relay to flash at standard rate all the time, even if a bulb goes.
https://www.buellxb.com/forum/showthrea ... lasher-modA