Looks like another die chip, caused by a die clashing:
Discovering the previous example today, we may have have figured out why certain die chips, happen on certain areas of the designs. With a different design on the reverse, this die chip will not be there. (You learned it here first)
So why did the die chip just show on the field? Answer hidden below. Keep in mind this is a clash. The outlines of the two designs, crashed into each other. So it was just the outlines of deeper devices, that the fields slammed into each other. Thus the only area of the chip is on the field, that the reverse die broke off of this die from that field area. Just highlight the hidden space between these to lines? The answer is a thinker on this one.