If it was a maintenance problem wouldn't you expect excessive wear and not what appears to be a melted spot at the very top of the cone? Usually I would guess that a lack of lubricant would simply gouge away the facing on the cone And I would also think that unless these cones were run loose as a goose that the ball bearings would never get anywhere near this section of the cone. And if the cones WERE that loose the facing of the cone would be abraded everywhere wouldn't it?

I say a manufacturers error for the following reason:

These components are machined from bar stock to the finished shape and then oven fired to harden the bearing surface. (You could also use cheaper steel and case harden them but they would wear rapidly.) This leaves the possibility of the top being machined too thin and the piece melting when fired. I would expect it to look as that does.