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Looks like some serious die flow wear.
That's my interpretation. The anomaly is plainly a feature of the die, especially considering the radical difference in metal flow characteristics inside and outside it. Looks like the die hardening process maybe got rushed or mismanaged, and as a result the die did not age evenly, but sank around that circle.
The design, with so little at the periphery and so much in the center, could have contributed to the look.