I can well believe that is the original sprocket. It certainly looks like it has 30,000 miles of wear and has the plastic cushioning to reduce noise… these are normally quite pricey (until Ognibene started offering them recently) so it’s not likely to have been changed.
My 12mm nut did the same thing a few months ago, it was still prevented from rotating by the lock washer, but appeared to have jumped one thread so the sprocket was able to slide slightly on the splines.
I don’t think you should beat yourself up about it. I torqued mine up correctly with threadlock too, the problem is nothing to do with undersized shafts (at least, not until they’ve been battered to death by a completely loose sprocket). Yamaha focced up by designing it with a completely unsuitable thread pitch, it’s far too fine for the sort of loads it experiences.
To get round this they applied a threadlock compound that was at the superglue end of the spectrum, hence the need for a 6ft breaker bar and the rifleshot sound effect when the original nut is undone. I suspect Yamaha had enough complaints about this from dealers that they switched to a weaker compound for later production.
Despite the extra turns of the deeper nut, the strength is marginal so it will eventually come loose. For most people the C&S will need changing before that point occurs provided you use a strong threadlock compound on a scrupulously cleaned thread.
I’d be interested to see a pic of the inner side of your sprocket, I noticed mine has heavy wear on that side so it is possible that a slight misalignment of the front and rear sprockets causes a sideways hammering effect on the front which might explain the nut getting bumped outwards.
