Hah! That's awesome! Best fix and nobody thought to ask. Thanks for letting us know.
I agree with Ray-UK. The LED lights are pulsed and the camera is catching the pulses during the read-out of the sensor (horizontal orientation)....
Like Ray-UK said, I suspect the flexible cable. I had the same problem with my EF 24-70L. It was an internal cable that the glue dried out on that...
I believe the camera door "disconnects" the battery when it is opened and reconnects it when it is closed. That's why the LED flashes right after...
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