Update: I installed Motion 4 on my 2010 MBP and rendered with the same settings.
Instead of the blocky artifacts, those replicator elements completely disappeared. However, besides that it was perfect.
I turns out, at least on my MBP, turning off the Bloom filter on the replicator’s source path brought back the missing replicator elements. I tried applying this fix to the MacPro, but the artifacts remained, making me think it is some shortcoming with floating points and/or the video card subsystem.