Any idea what this is? At first glance, looks like a deep scratch. Upon closer inspection, it has frost so it isn't a scratch. Thought maybe it was a cracked die, but it is incuse not raised as you would expect with a cracked die. Maybe a plancet error? If so, how do you explain the raised letters in the legend on top of it? Kind of weird... Note: Coin has medal rotation.
I see no reason why a planchet flaw would stop the metal from flowing into the recesses of the die (lettering). What is odd is that you have a huge remnant planchet flaw in the field of the coin. Typically planchet flaws remain in the deepest recesses of the die (eg. the jaw of a Jefferson Nickel). My guess is that the flaw on this planchet was much more severe before striking and what you are seeing is what is left. It shouldn't be too hard to strike away a planchet flaw in the field of a gold coin unless it was very deep.
Had thought perhaps that was the case, flow into the legend portion of the die to create letters on top of the flaw. The letter "M" is so well defined along the left-most portion of the letter, that it seems almost impossible to explain. Would think some of the flow would have created distortion in the shape of the letter, yet it is perfectly formed. Almost as if there might have been a lamination and upon being struck, the legend portion remained while the rest in the fields flaked off the planchet.