Cameron White

I believe there were some issues with displaying agents in the viewport on OSX, that were fixed in 16.5 and 16.0.817

The “Parallel Gauss-Seidel (Islands)” solver in 16.5 is essentially the same as the old single-threaded constraint solver, but if there are independent sets of interacting objects (islands) it will solve them in parallel. If you have a single big stack of objects, it will be single-threaded. You shouldn't see any significant change in behaviour, unless you were using the old parallel constraint solver.

Just a guess, but you may be affected by this fix in 16.5:

In 16.5.144:

Fixed a couple issues with how the Bullet solver reports the applied force / torque for a constraint. A bug in the Bullet engine caused the applied impulse to only be reported for one axis, and the values could change dramatically when adjusting the number of substeps on the solver. To update existing setups, force thresholds should likely be multiplied by at least the number of solver substeps.

For constraints that can affect position or rotation (e.g. pin and spring), the solver now uses significantly less memory and is about 20% faster solving constraints when the constraint type is set to “all”

The Bullet solver now always outputs the ‘force’, ‘torque’, ‘distance’, and ‘angle’ attributes regardless of constraint type.

Yes, there are optional array attributes that you can use to set properties of each joint's rigid body (see https://www.sidefx.com/docs/houdini/nodes/dop/rbdpackedobject#agentattributes)