0.14 amps (120v) circuit flowing through new #12 egc
I agree, they all must be bonded at very least at at the grounding electrode system. But if you have metal structure components or other metal items in contact with these there is easily inadvertent bonding likely to happen. And yes any jackleg neutral or even unintended faults at times can leave you with unintended current on your EGC's, metal raceways, and in non electrical related but conductive items.There is no benefit to trying to "isolate" conduit systems. As a practical matter you may not be able to as there are a lot of connections to the conduit systems via the building structure anyway. Plus they are all connected to the same grounding electrode system.
I don't know what to tell you about the small amount of current flowing on your EGC. I'd be inclined to track it down and fix it but chances are in an old system it will not be easy. It may be that somewhere some idiot installed a jackleg neutral.
0.14 amps (120v) circuit flowing through new #12 egc