Some chemical reaction to the galvanizing materials? Maybe. Galvanic corrosion would show on the metal first I would think and if the brick or rock is mettalic maybe some reaction to that but the morter? I don't think it is an electrical cause.
Common Portland cement [ now almost universal ] was invented in about 1825. I believe Edison had a cement works in about 1899 - and his usual famous 'backdated patents', no doubt! I'd guess that the original mortar was thus lime based, often with added straw, or animal hair, or wood ash and sand for economy. That stuff took months to set, but nobody was in a hurry in those days. The biggest problem with pointing up old masonary in patches is that there is a tendency to make the mortar too strong- this can crack the masonary- or to use up an old bag of cement in the truck that's gone off - mortar then crumbles. Mortar should be 1:5 [unless flaunching roof ridges, chimneys and the like when 1:3 can be employed], no stronger for brick/stone and made with fresh cement, clean water and clean sand. Small areas of pointing often dry too fast and need damping down for a day or so- or worse, they get frosted and don't gel properly and crumble as a weak mortar. There will be a local voltage around metal parts- cements are alkaline and thus form electrolytes. Damage is usually minimal if iron is galvanised or painted. Bare iron will swell alarmingly in contact with mortar, to the extent that it can fracture structural beams. A tar coat usually suffices to prevent surface corrosion- deep embedded stuff like rebar is susceptible unless there is at least 2" of 'cover'. ps; My pa was a 'brickie', old school. He used to bore us kids rigid for hours with talk of frogs, sharp-sand, slate damp-courses, soldiers etc! Some of it must have sunk in!