Probably not so much a bug as a missing feature but should XY billboarding of particles (as distinct from normal sprites) not be at least an option? It affects all particle types - smooth, round and square.
If for some strange reason you do want particles not to face the camera, set gl_billboard_particles to false. I can't imagine a reason for that, though. Particles are small and short-lived enough that the sprite cuts you see near 3D floors shouldn't be a real problem with them, and since they are supposed to be fully symmetrical it makes sense that they'd be billboarded.