Not a fault, it's how it's meant to do it. A good number of threads on it on here.
Basically you set your mirrors by having the switch on the right and adjust the right mirror (ignore what the left one is doing), then press the left switch and get your left mirror sorted. Now both are adjusted. Press the right switch and the idea is that if someone else drives the car then they just push the left/right button and both mirrors move inwards/outwards at the same time by the same amount therefore adjusting the mirrors in one go.
Simples.
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