Just joined the forum to report that I'm having the same mag drop problem.  I received two mags from Remington a while back after reporting numerous nose dives.  I numbered all 4 mags so I could keep track of any problems.  The nose dives are few and far between now, but failure to fully enter battery still occurs way too often.  A quick tap on the rear of the slide is all it takes, but not ideal.  Finally, 3 of the mags will drop, but do not seem to follow any particular pattern.  It's happened on the first shot and randomly during the mag.  I've verified the mag is secure.  I've watched my thumb to keep it away from the release.  But the only sure way to prevent it so far is to use my off hand under the mag.  I've returned the 3 mags to Remington so they can check them.  I'm not sure if they will simply replace them or send them back with a cure or explanation.  But I'm hopeful.

After following the comments on this forum for several months, and seeing many of the same issues that I am having with my R51, I decided it was time to share a few things.  Hopefully, I will eventually end up with a pistol that runs as smoothly as some report.