occurrence. In the manufacturing process, the base chip is notched where the inserts will be inserted (hence the term, "inserts").

Ideally, there is no play between the insert(s) and the notch into which they are inserted, but tolerances are never that exact. Moreover, the composition of the base and insert(s), can be slightly different. When all are placed into the clamshell (or cup) molds, pressure is applied as is heat. This allows everything to fuse with one another. Sometimes, the base material becomes softer before the insert(s) and if the insert has some play, then the base material will flow across the face (edge) of the insert and results in the "split insert" appearance you show.

Whew...

This happens enough to be termed as poor quality, rather than an error.