Stock tail light - or aftermarket unit?
Not sure if the bulb is a two element version or not.
I've had a similar thing happen on cars and replacing the bulb fixed it.
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2008 WR250X
Gearing: 13t - 48t
Power Commander 5 / PC-V
Airbox Door Removed - Flapper glued - AIS removed
FmF Q4
Bridgestone Battlax BT-003rs