I was wondering if anyone knows why having a name attribute, specifically as @name, throws off the bullet solver, giving wanky results like passing through the ground plane; While removing the @name attribute or even having a differently named attribute doesn't - the solver acts fine as expected.

I'm guessing somewhere within the network the rbd solver creates/makes use of the name attribute somewhere, so having it pre-existing throws it off?

since s@name is one of the essential attributes of bullet, it's important that it has correct value, otherwise constraints will not be able to find pieces they should attach to and other things …

in your case however (since you are not using constraints or other stuff dependent on s@name) the sim itself is fine
main problem is that after the sim RBD Bullet Solver SOP is using Transform Pieces SOP to apply the motion from simmed (potentially proxy) geo to RBD geo and that is matching pieces by s@name, so all of them will get the motion of the first rbd point found with that name

also there is many other attributes that bullet is able to use
https://www.sidefx.com/docs/houdini/nodes/dop/rbdpackedobject.html [www.sidefx.com]
so don't be surprised if the sim is behaving unexpectedly if you are using any of them without being aware how (or that) it affects the sim or internals of certain assets

Thanks Tomas for the confirmation - saving me time looking through the network to find it.