My August 2019 M3LR has dimming interior and external mirrors. I've had no issue with them, and it's a benefit when motorway driving. It's an occasional irritation when some headlights reflect off the bit around the edge of the area that dims.
I don't have anywhere dark I reverse into, my drive has a motion activated light, and I tend to use the camera when reversing on more than mirrors. I haven't noticed whether they stay dimmed when reversing.