Cameron White

The circle drawn in the viewport is derived from the current bounding box of the agent [www.sidefx.com] (e.g what you would get from the Bound SOP or the `bounds` primitive intrinsic

However, it's located at the agent's point position, not the bbox center, and its radius is set to half the X/Z size

That sounds like a bug if it's not terminating - are you able to submit that example skeleton to support? I wonder if there's something invalid in the hierarchy causing issues

Regardless, I think that particular node is actually unnecessary in this case since it's only required when the Agent Pose from Rig node has Root Transform set to "Transfer to Primitive Transform" and the Root Joint parameter is empty.

If the props are part of the agent's geometry, I think you'd want to make sure you have a separate prop joint so that the shield can move independently from the hand joint when it separates.
Then if you want to drop the shield via the ragdoll solver, you could switch to a partial ragdoll state where the prop joint is active but the rest of the character is following your animation clip. If the prop joint is parented to other joints in the skeleton you would also need to remove the Bullet constraint that's holding it together in the ragdoll sim (this is basically the same idea as detaching limbs from a ragdoll https://www.sidefx.com/docs/houdini/crowds/ragdoll.html#detaching-limbs-advanced)