I have the same exact problem, My brand new shimano deore 12s crank keeps falling off, but the day before it was installed and double checked by Acme Bicycles, and then it fell right off again. So i put it back on, and i was riding dirt jumps that day, and the crank fell off up the lip of the gap jump, and it flung me over the bars face first into the landing.It's fallen off 3 times now while riding. I have cleaned, regreased, and reinstalled it each time, but what is the problem? Nothing is stripped. It's the non drive size side. I bought the crank earlier in the summer and put about 500 miles on the crank so far.
I have sent a letter to Shimano 3 weeks ago, with no response so far. Any help/input is appreciated, thank you.