Why do particles disappear after the camera moves?

I forgot to mention we also have this very basic particle emitter when the beams connect from opposite sides.:
http://107.161.22.27/paste/images/3.png
and this has the same problem (going invisible), so I don’t think the beam source/target are the problem. This ‘ball’ particlesystem’ s position is set simply with SetWorldLocation( FVector v ), and like the beams, works great at the beginning of a scene.
As extra information, we create all the beams and ball in C++ constructors.

PCIP.CreateDefaultSubobject<UParticleSystemComponent>( this, TEXT("BeamParticles") );

and here’ s something interesting:
http://107.161.22.27/paste/particles2.webm

  1. players move away from the start zone, so we introduce the bug
  2. activate beam, but we can’t see it.
  3. Eject from game, beam is still invisible
  4. Move the camera outwards in the editor, et voila: the particles appear again.