And why are you incrementing them, when they aren’t ambiguous? Can you provide the instructions for this kind of style? Is it possible that the “blocks” are characters your font can’t handle? This is often the way “unknown” characters are displayed in word processors.
I am not sure that this is possible, but you might contact the tech support people directly about this facility. I don’t think the YEARa,b,c option was designed for this large a number of papers in a single year. If you have any leeway, I would use a numbered scheme rather than the author, year format for this kind of application.