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If my bike looked like that I would not be satisfied with these answers and would still be looking for a solution.
In the service manual it recommends swapping the exhaust pipes side to side every 12,000 miles
Pipes are different lengths.
As Pete alludes, the left (S) cylinder "works" harder than the right. Just the same, a good tuneup is in order. Start with adjusting the valves, TPS, throttle body sync in that order.
So my 2015 Griso with only 1200 miles on it already has more purple discoloring on the LHS vs. the RHS, which has some slight purpling. Wonder if I should be concerned or whether the mapping even can be adjusted for Left side vs. Right side...will have to ask Beetle on this.
Most likely the air screw is cracked open on that side to balance idle.