The microswitch very often become dislodged and cause issue with the folding operation. It may be worth popping off the mirror cover and checking. Easy to do. When I got my car, both mirrors would fold in (under power) but immediately fold back out again. It turned out to be a loose microswitch on one side. I popped it back into place and everything now works as it is supposed to.

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Wayne
2005 BMW X5 3.0d (b 02/05)
2001 BMW F650GS Dakar (b 06/01)