Thank you guys for your help!
Regarding the articulation root: You couldn’t add one because one was already present in the “world” prim.
I still rebuilt the scene and added the articulation root to one of the cubes like you mentioned, however with no improvement. You can find the new scene attached.
What I find interesting is that the JointState Publisher, targeted towards the same prim, works just fine.
Joint_test_V2.usd (7.9 KB)