Questions about rigid obstacles in "Learning to Simulate Complex Physics with Graph Networks"

Hi,
Thanks for sharing the work 'Learning to Simulate Complex Physics with Graph Networks". The results looks awesome and I'm trying to re-implement it myself.

However I came across a question about the rigid obstacles in the task WATER-RAMP, specifically the particles in these obstacles.
As far as I understand the edges in the constructed graph are directional and each pair of particles has two edges such that the information is propagated in both ways. This makes sense for fluid particles. However for the particles in the rigid obstacles, their positions, velocities and accelerations are supposed to remain constant, and therefore the latent representations as well. In the paper the authors mentioned that "Those particles are treated identical to regular particles, but they are masked out of the loss". So I'm curious why the information is propagated towards these fixed particles from particles of other materials. Wouldn't this change the latent representations of the obstacles particles at every propagation step and affect the other particles at the following propagation steps? Thanks in advance!