Since the reverse design is weak or absent opposite the divot on the obverse, I suspect this is a "rolling indentation" error. A piece of debris was rolled into the coin metal strip and fell out after rolling was completed. The impression of the object left the coin thinner in this area, which is why it didn't strike up properly. While such errors should be underweight, the small size of the indentation would not have created much of a weight difference. While I can't entirely rule out a bifacial grease strike, the depth and sharp outline of the obverse impression suggests the presence of a discrete piece of debris. Also, bifacial grease strikes usually lie closer to a coin's edge.
Error coin writer and researcher.