Hi zbyte. Which PSU was used with the most downstream (DAC side) FMC in your test? An SMPS?
I have a pair of TP-Links where I had the switching regulators removed and replaced with low noise linear regulators and I’m able to power the FMC’s with 5Vdc - including powerbanks… which means you are truely ‘off the grid’.
If using an SMPS (and even an un-grounded linear PSU) to power the last FMC, there is still a path for leakage currents to go through the DAC, according to John Swenson’s current personal research.