I experimented with locking the VoxelTree for collision queries although in the end I explicitly set the lock strategy to the default value, so the behavior is unchanged.

The fact that the VoxelTree sometimes returns valid collision info and sometimes does not (such as when it can't get the lock) causes problems.  In particular the avatar will sometimes bob down and up when the floor suddenly drops out from underfoot for a frame or two.  Locking the VoxelTree for these collision queries fixes the bobbing but seems to cause a less regular (and slightly lower average) render FPS.