Faded frames in animation

I’m dealing with an odd issue that I’ve replicated in multiple versions of Blender.


At a point in the animation, starting at frame 132 to be exact, the objects fade and become translucent, allowing the environment map to be visible. The frames where the objects are faded seems to be almost at random, with the objects being opaque, then faded, then opaque again.

I know that it has something to do with motion blur, because if I turn it off, the renders are fine. Likewise, it has something to do with the icosphere that I’m using as a particle, because if I get rid of that, the renders are fine.

Any input is much appreciated.