I've read about player sinking into the ground when deploying weapon, but this error seems slightly different. On the Stratis map, at the small town near the airbase, there's a street that cuts across the town (parallel to the overflow canal). At one end is an open garage near some concrete barriers. It you go prone in front of the garage opening (in the dead leaves), then use your bipod, your character's body takes an odd angle and nearly disappears under the concrete.