This electrolysis issue has been known for many years. This is why (at some point in the design) Subaru added additional grounding cables from each head to the chassis.
If you think about it.... each time a sparkplug fires there needs to be a COMPLETE CIRCUIT from the ignition-coil, to the sparkplug, BACK TO THE COIL.
Most of that high-voltage is passing through the threads on the head-bolts. Eventually the threads corrode and become an insulator... this forces the current THRU THE ANTIFREEZE pinched between the head-gaskets.
It is a VERY good idea to instead provide a current-path from each head back to the ignition-coil by adding wires.
If one has to replace head-gaskets... make CERTAIN that the wires are re-installed. (from heads to engine-block AND from head to chassis).... lest your new head-gaskets will not live very long.