We might need you to describe in greater detail what your trying to do and how your trying to do it, but Ill take a stab in the dark.
Your appling a mask to a solid with Particular on it. It does not work that way Im guessing because particular creates it's own 3D math inside the plugin and overlays that information on a 2D AE layer. Most of the "illusion of 3D" effects do not respect the typical render order rules.
There are other methods like obscuration and lumma mats that might be more appropriate for what your trying to do. What are you trying to do?
I made some fake cigarettes cause none of my actor can actually smoke and I've added the smoke in post. Now when the cigarette goes in front of them or gets blocked by something I need to mask or hide where the particles are emitting from but still have them emit. I'll give a shot at what Brian says to do and see if that works.
I would probably approach this by duplicate my footage and masking the charector rather than the particles. Place the foreground elements on top of the particles in your stacking order.
The problem with precomposing particular layers is that you lose the connection between your particles and camera. This connection is necessary for accurate motion blur and depth of field. Probably not an issue with your comp but something to keep in mind.