Just a comment regarding injection molded plastic parts in general. When plastic flows into the mold, flow splits into 2 areas and ultimately joins somewhere to complete the filling of the cavity. The junction of the 2 areas is called a knit line. If there is too little pressure or the plastic/mold too cool, the knit line can be very weak. While not as likely with jockey wheels, if there is a stress riser like a sharp corner near the knit line, a crack can easily develop there.
The knit line/molding precess parameters could explain the crack.
Gary Watts Vancouver, Washington USA
"I have in front of me 3 NR jockey wheels that all have the identical hairline crack running from the center metal insert to one of the teeth. I cleaned them really well in rubbing alcohol and filled the crack with JB weld brand 2 part epoxy. Will that save them for future service?"
Garth Libre in Miami Fl USA