I've had an M12V for a few years and found almost immediately that I wasn't comfortable with the slide lock and only one wrench. Initially I had the M12V mounted in my router table. My concerns were realized when a large round over walked out of the collet. Luckly no one was hurt. Since then I have used 2 wrenches to tighten the collet. The second wrench needed is an odd size (23 MM) and hard to find so I made one. Since then I've had no problems with bits, 1/2" or 1/4" shank, walking out. I was too embarassed to call Hitachi and complain.
I believe using 2 wrenches is a more permanent fix to the loose bit problem.
Good luck with it,