I can't figure out why the only object with which the particles collide is the sphere. All three objects are set as polygon mesh. I have tried each one individually, and only the sphere is detected by the particles.

It makes no sense as in this other scene i just finished, i used a massive amount of particles hitting all sorts of crazy geometry, in a mix of animated alembic, FBX and primitives with no trouble whatsoever: https://fabiobasile.myportfolio.com/rain-short-animation [fabiobasile.myportfolio.com]

I feel i am missing something really obvious… any help is appreciated.

Solved by going into RBD Solver, and select “Use Surface Collisions” from the Collision Detection dropdown menu.

Live and learn…