nope, not strange.
you may have some stray current but it does happen.

2 dissimilar metals in a current carrying solution will get galvanic corosion.

aluminum corrodes faster than cast iron and it will eventually eat it away.

extra ground helps, having rad fluid changed out to something that doesn't carry current helps. Hopefully they fully flushed your system with new coolant.

also I added a zinc annode to my system to so it will be eaten first.
Change it out every few years.

Same thing happens in your hot water heater,
same thing happens on boats/ships etc..

got mine from some place called boatzincs or something like that.
put it in a block plug instead of the rad but either way works.

gives the ssytem something else to eat instead of the rad.