I think it's a long-standing confusion, not your memory. I
definitely read that it was arsenic in apple seeds many, many years ago. And I'm quite certain I
never read about cyanide in apple seeds. I have no reason why I would mix those two things up. To me the first thing that comes to mind for arsenic is a solid powder, and cyanide as "cyanide gas". Arsenic was common rat poison, cyanide was this high-tech thing they only talked about in spy movies. Not necessarily accurate, idk, but those are the strong mental associations I have of those two things from my childhood. There's no way I'd have mixed up a report about cyanide with arsenic.
It's not
memory - the sources were just wrong.
I think the reason for this is that in the English-speaking world, arsenic was a more familiar poison, so when people heard that there's poison in apple seeds, almonds etc, that got mutated into "arsenic in apple seeds".
However here's an important point:
https://en.wikipedia.org/wiki/Arsenic#ApplicationsArsenic was also used in various agricultural insecticides and poisons. For example, lead hydrogen arsenate was a common insecticide on fruit trees,[54] but contact with the compound sometimes resulted in brain damage among those working the sprayers.
So it's possible that this was in fact true, but it got conflated with the cyanide in seeds thing later on, to become "arsenic in apple seeds".