It fell off when being moved to the "down" position, I believe the tensioning spring was not installed properly (at least it looked that way) or the lever itself was not seated where it should have been. I have 2 other M&P pistols and neither have ever had the problem.
Just bought one, when getting ready to clean before first use the Sear Release Lever fell out when I moved it. Now waiting on S&W to repair it (and they are on 2 week shutdown - UGH)