Since the acid attacks all the surfaces equally the details all remain and remain clear the whole time even down to the point where the coin is paper thin. The one thing that WILL disappear is the rim because the acid attacks the edge of the coin as wellso the rim gets thinner and thinner as the diameter of the coin is reduced. That has happened on your cent. The diameter has been reduced,and the rim is gone or almost gone, but the dentils inside the rime are still there. If the acid soaking continued the dentils would get shorter as the diameter reduced further until they disappeared as well.