Happens all of the time to with any type of gun. It's by far the most annoying bug in the game at the moment. The weapon often doesn't go away immediately, but after a second or 2. Only after I run around for a minute or 2, and/or raise/lower my fist will I be able to successfully keep my weapon out in my hands.

I love it when I see a player or zed, line them up for a shot, and the weapon goes back into the holster.