Is there a bug when setting two groups of particles and their interaction with mesh walls

Dear all,

I am facing a strange problem about particles go through the stl-mesh walls. In my case, I want the small particles (atom type 1) to go through the walls while the big particles (atom type 2) remained inside of the walls. Therefore, I set two groups for these two particles and then use "fix wall/gran" to set the interaction between the big particles and the walls. I didn't set any interaction between small particles and walls, thus the small particles should go through the walls directly. However the results show that both the big and small particles are prevented by the walls. I have struggled with this problem several days but have no solutions. Could someone help me with this problems? How to make the small particles go through the walls while the big particles remain inside of the walls? Many thanks.

I have attached an easy example for all of you interested in this post to have a test. I hope to know your ideas. Thanks.

Regards,
Zheng